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ISL6406, ISL6426
Data Sheet November 1, 2004 FN9073.6
Single Synchronous Buck Pulse-Width Modulation (PWM) Controller
The ISL6406, ISL6426 is an adjustable frequency, synchronous buck switching regulator optimized for generating lower voltages for the distributed DC-DC architectures. The ISL6406 offers an adjustable output voltage, while the ISL6426 provides a fixed 1.8V output. Designed to drive N-Channel MOSFETs in synchronous buck topology, the ISL6406, ISL6426 integrates the control, output adjustment and protection functions into a single package. The ISL6406, ISL6426 provides simple, single feedback loop, voltage-mode control with fast transient response. The output voltage can be precisely regulated to as low as 0.8V. The error amplifier features a 15MHz gain-bandwidth product and 6V/s slew rate which enables high converter bandwidth for fast transient performance. Protection from overcurrent conditions is provided by monitoring the rDS(ON) of the upper MOSFET to inhibit PWM operation appropriately. This approach simplifies the implementation and improves efficiency by eliminating the need for a current sense resistor. The wide programmable switching frequency range of 100kHz to 700kHz allows the use of small surface mount inductors and capacitors. The device also provides external frequency synchronization making it an ideal choice for DC-DC converter applications.
Features
* Operates from 3.3V/5V Input * 0.8V to VIN Output Range - 0.8V Internal Reference - 1.5% Reference Accuracy * Simple Single-Loop Control Design - Voltage-Mode PWM Control * Fast Transient Response - High-Bandwidth Error Amplifier * Lossless, Programmable Overcurrent Protection - Uses Upper MOSFET's rDS(on) * Programmable Switching Frequency 100kHz-700kHz * External Frequency Synchronization * Two Device Options Available - ISL6406 . . . . . . . . . . . . . . . . Adjustable Output Voltage - ISL6426 . . . . . . . . . . . . . . . . . . . . . . Fixed 1.8V Output * Internal Soft-Start * QFN Package Option - QFN Compliant to JEDEC PUB95 MO-220 QFN - Quad Flat No Leads - Product Outline - QFN Near Chip Scale Package Footprint; Improves PCB Efficiency, Thinner in Profile * Pb-Free Packaging Available (RoHS Compliant) - Designated with "Z" Suffix (Refer to Note)
Applications
* 3V/5V DC-DC Converter Modules * Distributed DC-DC 3.3V, 2.5V and 1.8V Power Architectures for DSP, Logic, and Memory * Power Supplies for Microprocessors - PCs - Embedded Controllers * Memory Supplies * Personal Computer Peripherals
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 321-724-7143 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright (c) Intersil Americas Inc. 2003-2004. All Rights Reserved All other trademarks mentioned are the property of their respective owners.
ISL6406, ISL6426 Pinouts
ISL6406, ISL6426 (SOIC/TSSOP) TOP VIEW
GND 1 LGATE 2 16 UGATE 15 BOOT 14 PHASE 13 VCC 12 CPGND 11 COMP 10 VOUT 9 FB CPVOUT OCSET CT1 CT2
ISL6406, ISL6426 (QFN) TOP VIEW
UGATE 14 LGATE BOOT 13 12 PHASE 11 VCC 10 CPGND 9 5 RT 6 SYNC/EN 7 FB 8 VOUT COMP GND 15
CPVOUT 3 OCSET 4 CT1 5 CT2 6 RT 7 SYNC/EN 8
16 1 2 3 4
Ordering Information
PART NUMBER ISL6406CB ISL6406CBZ (See Note) ISL6406IB ISL6406IBZ (See Note) ISL6406CR ISL6406CRZ (See Note) ISL6406IR ISL6406IRZ (See Note) ISL6406CV ISL6406CVZ (See Note) ISL6406IV ISL6406IVZ (See Note) ISL6426CB ISL6426CBZ (See Note) ISL6426IB ISL6426IBZ (See Note) ISL6426CR ISL6426CRZ (See Note) ISL6426IR TEMP. RANGE (C) 0 to 70 0 to 70 -40 to 85 -40 to 85 0 to 70 0 to 70 -40 to 85 -40 to 85 0 to 70 0 to 70 -40 to 85 -40 to 85 0 to 70 0 to 70 -40 to 85 -40 to 85 0 to 70 0 to 70 -40 to 85 PACKAGE 16 Ld SOIC 16 Ld SOIC (Pb-free) 16 Ld SOIC 16 Ld SOIC (Pb-free) PKG. DWG. # M16.15 M16.15 M16.15 M16.15
Ordering Information
PART NUMBER ISL6426IRZ (See Note) ISL6426CV ISL6426CVZ (See Note) ISL6426IV ISL6426IVZ (See Note) TEMP. RANGE (C) -40 to 85 0 to 70 0 to 70 -40 to 85 -40 to 85 PACKAGE PKG. DWG. #
16 Ld 5x5 QFN L16.5x5B (Pb-free) 16 Ld TSSOP 16 Ld TSSOP (Pb-free) 16 Ld TSSOP 16 Ld TSSOP (Pb-free) M16.173 M16.173 M16.173 M16.173
16 Ld 5x5 QFN L16.5x5B 16 Ld 5x5 QFN L16.5x5B (Pb-free) 16 Ld 5x5 QFN L16.5x5B 16 Ld 5x5 QFN L16.5x5B (Pb-free) 16 Ld TSSOP 16 Ld TSSOP (Pb-free) 16 Ld TSSOP 16 Ld TSSOP (Pb-free) 16 Ld SOIC 16 Ld SOIC (Pb-free) 16 Ld SOIC 16 Ld SOIC (Pb-free) M16.173 M16.173 M16.173 M16.173 M16.15 M16.15 M16.15 M16.15
Add "-T" suffix to part number for tape and reel packaging. NOTE: Intersil Pb-free products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate termination finish, which are RoHS compliant and compatible with both SnPb and Pb-free soldering operations. Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020C.
16 Ld 5x5 QFN L16.5x5B 16 Ld 5x5 QFN L16.5x5B (Pb-free) 16 Ld 5x5 QFN L16.5x5B
2
ISL6406, ISL6426 Functional Block Diagram
VCC CPVOUT
CT1 CT2 CPGND OCSET 20A
CHARGE PUMP
SDWN
POWER-ON RESET (POR) BOOT SOFTSTART UGATE
OC COMPARATOR
+ -
PHASE ERROR AMP + PWM COMPARATOR + SDWN FB VOUT COMP OSCILLATOR INHIBIT
+
0.8V
GATE CONTROL LOGIC PWM
LGATE
SYNC/EN
RT
GND
3
ISL6406, ISL6426 Typical Application Schematic for 5V Input
VIN 5V 10% CIN VCC CT1 OCSET ROCSET CPVOUT ISL6406, ISL6426 RT CPGND RT GND BOOT CBOOT UGATE PHASE VCC VOUT SYNC/EN COMP LGATE FB Q2 COUT Q1 LOUT DBOOT CBULK
NC
CT2
CDCPL CHF RBOOT
VOUT
CI RFB RF CF ROFFSET
Typical Application Schematic for 3.3V Input
VIN 3.3V 10% CIN VCC CT1 CPUMP CT2 RT CPGND RT GND OCSET ROCSET CPVOUT ISL6406, ISL6426 BOOT CBOOT UGATE PHASE VCC VOUT SYNC/EN COMP LGATE FB Q2 COUT Q1 LOUT DBOOT CDCPL CHF RBOOT CBULK
VOUT
CI RFB RF CF ROFFSET
4
ISL6406, ISL6426
Absolute Maximum Ratings
Supply Voltage, VCC (Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . +7.0V Absolute Boot Voltage, VBOOT . . . . . . . . . . . . . . . . . . . . . . . +15.0V Upper Driver Supply Voltage, VBOOT - VPHASE . . . . . . . . . . . +6.0V Input, Output or I/O Voltage . . . . . . . . . . . GND -0.3V to VCC +0.3V ESD Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Class 2
Thermal Information
Thermal Resistance (Typical) JA (C/W) JC (C/W) SOIC (Note 2) . . . . . . . . . . . . . . . . . . . 70 N/A TSSOP (Note 2) . . . . . . . . . . . . . . . . . . 90 N/A QFN (Notes 3, 4) . . . . . . . . . . . . . . . . . 35 4.5 Maximum Junction Temperature (Plastic Package) . . -55C to 150C Maximum Storage Temperature Range . . . . . . . . . . . -65C to 150C Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . . 300C (SOIC - Lead Tips Only)
Operating Conditions
Temperature Range ISL6406, ISL6426C . . . . . . . . . . . . . . . . . . . . . . . . . . 0C to 70C ISL6406, ISL6426I . . . . . . . . . . . . . . . . . . . . . . . . . .-40C to 85C Supply Voltage Range . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3V 10%
CAUTION: Stresses above those listed in "Absolute Maximum Ratings" may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
NOTES: 1. Please refer to the Typical Application Schematics (page 3) for 3.3V / 5V input configuration. 2. JA is measured with the component mounted on a high effective thermal conductivity test board in free air. See Tech Brief TB379 for details. 3. JA is measured in free air with the component mounted on a high effective thermal conductivity test board with "direct attach" features. See Tech Brief TB379. 4. For JC, the "case temp" location is the center of the exposed metal pad on the package underside.
Electrical Specifications
PARAMETER VCC SUPPLY Shutdown Supply Current Operating Supply Current (Note 5) REFERENCE VOLTAGE Nominal Reference Voltage Reference Voltage Tolerance
Recommended operating conditions unless otherwise noted. Refer to Block Diagram and Typical Application Schematic. VCC = +3.3V. Typical values are at TA = 25C. TEST CONDITIONS MIN TYP MAX UNITS
SYNC/EN = GND RT = 64.9k
7
20 9.8
50 11.5
A mA
-1.5 TA = 0C to 70C TA = -40C to +85C -1.8 -2.1
0.8 -
1.5 1.8 2.1
V % % %
ERROR AMPLIFIER Open Loop Voltage Gain (Note 6) Gain-Bandwidth Product (Note 6) Slew Rate (Note 5) CHARGE PUMP Nominal Charge Pump Output Charge Pump Output Regulation POWER-ON RESET Rising CPVOUT POR Threshold TA = 0C to 70C TA = -40C to +85C CPVOUT POR Threshold Hysteresis OSCILLATOR Gate Output Frequency Range RT = 200k RT = 64.9k RT = 26.1k Sawtooth Amplitude Peak-to-Peak VOSC 80 250 650 1.1 100 300 715 1.4 120 340 770 1.7 kHz kHz kHz V 4.20 4.1 0.3 4.35 4.35 0.5 4.5 4.6 0.9 V V V VCC = 3.3V, No Load 4.8 -5.0 5.1 5.5 5.0 V % COMP = 10pF 14 4.65 82 6.0 9.2 dB MHz V/s
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ISL6406, ISL6426
Electrical Specifications
PARAMETER Sync. Frequency Range (Note 6) Minimum Sync Pulse Width (Note 6) PWM Maximum Duty Cycle GATE DRIVER OUTPUT (Note 6) Upper Gate Source Current Upper Gate Sink Current Lower Gate Source Current Lower Gate Sink Current SOFT-START Soft-Start Slew Rate f = 300kHz, TA = 0C to 70C f = 300kHz, TA = -40C to +85C Internal Digital Circuit Clock Count (Soft-start time varies with frequency) OVERCURRENT OCSET Current Source TA = 0C to 70C TA = -40C to +85C NOTES: 5. This is the VCC current consumed when the device is active but not switching. 6. Guaranteed by design. 18 16 20 20 22 23 A A 6.2 6.2 6.7 6.7 2048 7.3 7.6 ms ms Clk Cycles VVCC = 3.3V, VLGATE = 4V VBOOT - VPHASE = 5V, VUGATE = 4V -1 1 -1 2 A A A A Recommended operating conditions unless otherwise noted. Refer to Block Diagram and Typical Application Schematic. VCC = +3.3V. Typical values are at TA = 25C. (Continued) TEST CONDITIONS 1.1 Times the natural switching frequency. MIN 110 TYP 40 96 MAX 770 100 UNITS kHz ns %
Typical Performance Curve
0.81 0.805 0.8 VREF (V) 0.795 0.79 0.785 0.78 -40 -30 -20 -10
0
10
20
30
40
50
60
70
80
TEMPERATURE (C)
FIGURE 1. REFERENCE VOLTAGE vs TEMPERATURE
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ISL6406, ISL6426 Pin Descriptions
CPVOUT - This pin represents the output of the charge pump. The voltage at this pin is the bias voltage for the IC. Connect a decoupling capacitor from this pin to ground. The value of the decoupling capacitor should be at least 10x the value of the charge pump capacitor. This pin may be tied to the bootstrap circuit as the source for creating the BOOT voltage. CT1 and CT2 - These pins are the connections for the external charge pump capacitor. A minimum of a 0.1F ceramic capacitor is recommended for proper operation of the IC. OCSET - Connect a resistor (ROCSET) from this pin to the drain of the upper MOSFET (VIN). ROCSET, an internal 20A current source (IOCSET), and the upper MOSFET onresistance (rDS(ON)) set the converter overcurrent (OC) trip point according to the following equation:
( I OCSET ) ( R OCSET ) I PEAK = ------------------------------------------------------r DS ( ON )
BOOT - This pin provides ground referenced bias voltage to the upper MOSFET driver. A bootstrap circuit is used to create a voltage suitable to drive a logic-level N-Channel MOSFET. A large (~1MOhm) resistor should be connected from this pin to GND. The purpose of this resistor is to discharge the BOOT pin during a shutdown condition, SYNC/EN = LOW so that the gate drivers are quickly powered off by this bleed resistor. UGATE - Connect this pin to the upper MOSFET's gate. This pin provides the PWM-controlled gate drive for the upper MOSFET. This pin is also monitored by the adaptive shootthrough protection circuitry to determine when the upper MOSFET has turned off. GND - This pin represents the signal and power ground for the IC. Tie this pin to the ground island/plane through the lowest impedance connection available. LGATE - Connect this pin to the lower MOSFET's gate. This pin provides the PWM-controlled gate drive for the lower MOSFET. This pin is also monitored by the adaptive shootthrough protection circuitry to determine when the lower MOSFET has turned off. COMP and FB - COMP and FB are the available external pins of the error amplifier. The FB pin is the inverting input of the internal error amplifier and the COMP pin is the error amplifier output. These pins are used to compensate the control feedback loop of the converter. CPGND - This pin represents the signal and power ground for the charge pump. Tie this pin to the ground island/plane through the lowest impedance connection available. SYNC/EN - This is a dual-function pin. To synchronize with an external clock, apply a clock with a frequency 1.1 to 2.0 times higher than the part's natural frequency to this pin. The device may be disabled by tying this pin to ground. In this shutdown mode, all functions are disabled and the device will draw <50A supply current.
An overcurrent trip cycles the soft-start function. VOUT - This pin provides the external switcher output voltage to the IC as feedback for the 1.8V fixed output voltage option. Tie this pin to 1.8V for the ISL6426 fixed 1.8V option. Leave this pin open on the ISL6406 for the adjustable output voltage option. VCC - This pin provides bias supply for the ISL6406, ISL6426. Connect a well-coupled 3.3V supply to this pin. PHASE - Connect this pin to the upper MOSFET's source. This pin is used to monitor the voltage drop across the upper MOSFET for overcurrent protection. RT - Connect an external resistor from this pin to ground for frequency selection. Refer to RT vs Frequency curve of Figure 3.
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ISL6406, ISL6426 Functional Description
Initialization
The ISL6406 automatically initializes upon receipt of power. Special sequencing of the input supplies is not necessary. The Power-On Reset (POR) function continually monitors the the output voltage of the charge pump. During POR, the charge pump operates on a free running oscillator. Once the POR level is reached, the charge pump oscillator is synched to the PWM oscillator. The POR function also initiates the soft-start operation after the charge pump output voltage exceeds its POR threshold.
900 800 700 600 500 400 300 200 100 0 200 180 150 130 100 80 64.5 50 44.5 39 33.0 26.4 22 RT (k) FREQUENCY (kHz)
Soft-Start
The POR function initiates the digital soft-start sequence. The PWM error amplifier reference is clamped to a level proportional to the soft-start voltage. As the soft-start voltage slews up, the PWM comparator generates PHASE pulses of increasing width that charge the output capacitor(s). This method provides a rapid and controlled output voltage rise. The soft start sequence typically takes about 6.5ms.
FIGURE 3. FREQUENCY vs RT
Shoot-Through Protection
A shoot-through condition occurs when both the upper MOSFET and lower MOSFET are turned on simultaneously, effectively shorting the input voltage to ground. To protect the regulator from a shoot-through condition, the ISL6406, ISL6426 incorporates specialized circuitry which insures that the MOSFETs are not ON simultaneously. The adaptive shoot-through protection utilized by the ISL6406, ISL6426 looks at the lower gate drive pin, LGATE, and the upper gate drive pin, UGATE, to determine whether a MOSFET is ON or OFF. If the voltage from UGATE or from LGATE to GND is less than 0.8V, then the respective MOSFET is defined as being OFF and the other MOSFET is turned ON. This method of shoot-through protection allows the regulator to sink or source current. Since the voltage of the lower MOSFET gate and the upper MOSFET gate are being measured to determine the state of the MOSFET, the designer is encouraged to consider the repercussions of introducing external components between the gate drivers and their respective MOSFET gates before actually implementing such measures. Doing so may interfere with the shoot-through protection.
(1V/DIV)
CPVOUT (5V)
VCC (3.3V)
VOUT (2.50V)
0V t0 t1 t2 t3 TIME
FIGURE 2. SOFT-START INTERVAL
Figure 2 shows the soft-start sequence for a typical application. At t0, the +3.3V VCC voltage starts to ramp. At time t1, the Charge Pump begins operation and the +5V CPVOUT IC bias voltage starts to ramp up. Once the voltage on CPVOUT crosses the POR threshold at time t2, the output begins the soft-start sequence. The triangle waveform from the PWM oscillator is compared to the rising error amplifier output voltage. As the error amplifier voltage increases, the pulse-width on the UGATE pin increases to reach the steady-state duty cycle at time t3.
Output Voltage Selection
The output voltage can be programmed to any level between VIN and the internal reference, 0.8V. An external resistor divider is used to scale the output voltage relative to the reference voltage and feed it back to the inverting input of the error amplifier, see Figure 4. However, since the value of R1 affects the values of the rest of the compensation components, it is advisable to keep its value less than 5K. R4 can be calculated based on the following equation:
( R1 ) ( 0.8V ) R4 = -----------------------------------------V OUT1 - ( 0.8V )
Frequency Selection
The ISL6406 offers adjustable frequency from 100kHz to 700kHz by changing external resistor connected at pin RT. Figure 3 shows the typical RT vs Frequency variation curve.
If the output voltage desired is 0.8V, simply route the output back to the FB pin through R1, but do not populate R4.
8
ISL6406, ISL6426
When using the fixed 1.8V output ISL6426 option, the internal resistor values are R1 = 3.5k and R2 = 2.8k, where R1 is connected from VOUT to FB and R2 is connected from FB to GND.
+3.3V
Overcurrent Protection
The overcurrent function protects the converter from a shorted output by using the upper MOSFET on-resistance, rDS(ON), to monitor the current. This method enhances the converter's efficiency and reduces cost by eliminating a current sensing resistor. The over current function cycles the soft-start function in a hiccup mode to provide fault protection. A resistor (ROCSET) programs the over current trip level (see Typical Application diagrams). An internal 20A (typical) current sink develops a voltage across ROCSET that is referenced to VIN. When the voltage across the upper MOSFET (also referenced to VIN) exceeds the voltage across ROCSET, the over current function initiates a soft-start sequence.
VCC CPVOUT D1 BOOT C4 UGATE PHASE LGATE
VIN
Q1
LOUT
ISL6406, ISL6426
VOUT
Q2
+ COUT
VOUT (2.5V)
FB COMP C1 R1 C3 C2 R2 R4 0V R3
FIGURE 4. OUTPUT VOLTAGE SELECTION
INTERNAL SOFT-START FUNCTION
Frequency Synchronization and Enable
The external frequency synchronization and enable functions are combined in SYNC/EN pin. This pin is TTL compatible for Vcc = 3.3V or 5V. The device is disabled if the input to this pin is TTL LOW for more than 40s (typ.); it is enabled if the input is TTL HIGH without delay. When disabling the IC, the charge pump is turned off and the BOOT pin is left charged at ~5V. In some cases this charge will inadvertant leak through the upper gate driver and can possibly turn on the upper FET. To avoid this, it is recommended that a 1MOhm `bleed' resistor be connected from the BOOT pin to GND. This resistor is shown in the typical application schematic in Typical Application Schematics as RBOOT. The SYNC/EN pin is monitored by the internal timer. The timer allows SYNC pulses (TTL LOW level) to pass through, as long as the pulses are shorter than 22s. The minimum SYNC pulse width is 40ns (typ.). The oscillator can SYNC to an external frequency of between 1.1 times and 2.0 times the free-running frequency. Loop acquisition time is about 200 clock cycles. The timing resistor (RT) is always required, regardless of whether SYNC pulses are being used or not. For instance, if RT is selected such that the switching frequency is 100kHz then the ISL6406 can be synchronized to a switching frequency from 110kHz to 200kHz.
DELAY INTERVAL
t0
t1 TIME
t2
FIGURE 5. OVERCURRENT PROTECTION RESPONSE
Figure 5 illustrates the protection feature responding to an overcurrent event. At time t0, an overcurrent condition is sensed across the upper MOSFET. As a result, the regulator is quickly shutdown and the internal soft-start function begins producing soft-start ramps. The delay interval seen by the output is equivalent to three soft-start cycles. The fourth internal soft-start cycle initiates a normal soft-start ramp of the output, at time t1. The output is brought back into regulation by time t2, as long as the overcurrent event has cleared. Had the cause of the over current still been present after the delay interval, the over current condition would be sensed and the regulator would be shut down again for another delay interval of three soft-start cycles. The resulting hiccup mode style of protection would continue to repeat indefinitely. The overcurrent function will trip at a peak inductor current (I peak) determined by:
( I OCSET ) ( R OCSET ) I PEAK = ------------------------------------------------------r DS ( ON )
9
ISL6406, ISL6426
where IOCSET is the internal OCSET current source (20A typical). The OC trip point varies mainly due to the MOSFET rDS(ON) variations. To avoid overcurrent tripping in the normal operating load range, find the ROCSET resistor from the equation above with: 1. The maximum rDS(ON) at the highest junction temperature. 2. The minimum IOCSET from the specification table. 3. Determine IPEAK for, IPEAK > IOUT(MAX) + (DI/2) where DI is the output inductor ripple current. For an equation for the ripple current see the section under Component Selection Guidelines titled Output Inductor Selection. A small ceramic capacitor should be placed in parallel with ROCSET to smooth the voltage across ROCSET in the presence of switching noise on the input voltage. switching interval. Careful component selection, tight layout of the critical components, and short, wide traces minimizes the magnitude of voltage spikes. There are two sets of critical components in a DC-DC converter using the ISL6406, ISL6426. The switching components are the most critical because they switch large amounts of energy, and therefore tend to generate large amounts of noise. Next are the small signal components, which connect to sensitive nodes or supply critical bypass current and signal coupling. A multi-layer printed circuit board is recommended. Figure 6 shows the connections of the critical components in the converter. Note that capacitors CIN and COUT could each represent numerous physical capacitors.
+3.3V VIN ISL6406 VCC CVCC
Current Sinking
The ISL6406, ISL6426 incorporates a MOSFET shootthrough protection method which allows a converter to sink current as well as source current. Care should be exercised when designing a converter with the ISL6406, ISL6426 when it is known that the converter may sink current. When the converter is sinking current, it is behaving as a boost converter that is regulating its input voltage. This means that the converter is boosting current into the input rail of the regulator. If there is nowhere for this current to go, such as to other distributed loads on the rail or through a voltage limiting protection device, the capacitance on this rail will absorb the current. This situation will allow the voltage level of the input rail to increase. If the voltage level of the rail is boosted to a level that exceeds the maximum voltage rating of any components attached to the input rail, then those components may experience an irreversible failure or experience stress that may shorten their lifespan. Ensuring that there is a path for the current to flow other than the capacitance on the rail will prevent this failure mode.
CPVOUT CBP GND BOOT CBOOT UGATE PHASE LGATE PHASE Q2 Q1 LOUT VOUT LOAD D1 CIN
COUT
COMP
C2 R2
C1 R1 R4 C3 R3
Application Guidelines
Layout Considerations
Layout is very important in high frequency switching converter design. With power devices switching, the resulting current transitions from one device to another cause voltage spikes across the interconnecting impedances and parasitic circuit elements. These voltage spikes can degrade efficiency, radiate noise into the circuit, and lead to device overvoltage stress. Careful component layout and printed circuit board design minimizes the voltage spikes in the converters. As an example, consider the turn-off transition of the PWM MOSFET. Prior to turn-off, the MOSFET is carrying the full load current. During turn-off, current stops flowing in the MOSFET and is picked up by the lower MOSFET. Any parasitic inductance in the switched current path generates a large voltage spike during the
KEY
FB
ISLAND ON POWER PLANE LAYER ISLAND ON CIRCUIT PLANE LAYER VIA CONNECTION TO GROUND PLANE
FIGURE 6. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS
Dedicate one solid layer, usually a middle layer of the PC board, for a ground plane and make all critical component ground connections with vias to this layer. Dedicate another solid layer as a power plane and break this plane into smaller islands of common voltage levels. Keep the metal runs from the PHASE terminals to the output inductor short. The power plane should support the input power and output
10
ISL6406, ISL6426
power nodes. Use copper-filled polygons on the top and bottom circuit layers for the phase nodes. Use the remaining printed circuit layers for small signal wiring. The wiring traces from the GATE pins to the MOSFET gates should be kept short and wide enough to easily handle the 1A of drive current. The switching components should be placed close to the ISL6406, ISL6426 first. Minimize the length of the connections between the input capacitors, CIN, and the power switches by placing them nearby. Position both the ceramic and bulk input capacitors as close to the upper MOSFET drain and islands as possible. Position the output inductor and output capacitors between the upper and lower MOSFETs and the load. The critical small signal components include any bypass capacitors, feedback components, and compensation components. Position the bypass capacitor, CBP, close to the VCC pin with a via directly to the ground plane. Place the PWM converter compensation components close to the FB and COMP pins. The feedback resistors for both regulators should also be located as close as possible to the relevant FB pin with vias tied straight to the ground plane as required.
OSC PWM COMPARATOR VOSC +
DRIVER
VIN LO VOUT CO
DRIVER
PHASE
ZFB VE/A + ERROR AMP ZIN REFERENCE
ESR (PARASITIC)
DETAILED COMPENSATION COMPONENTS C1 C2 R2 ZFB ZIN C3 R1 FB R3 VOUT
COMP + ISL6406, ISL6426 REFERENCE
Feedback Compensation
Figure 7 highlights the voltage-mode control loop for a synchronous-rectified buck converter. The output voltage (VOUT) is regulated to the Reference voltage level. The error amplifier (Error Amp) output (VE/A) is compared with the oscillator (OSC) triangular wave to provide a pulse-width modulated (PWM) wave with a peak amplitude of VIN at the PHASE node. The PWM wave is smoothed by the output filter (L and CO).The modulator transfer function is the small-signal transfer function of VOUT/VE/A. This function is dominated by a DC Gain and the output filter (LO and CO), with a double pole break frequency at FLC and a zero at FESR. The DC Gain of the modulator is simply the input voltage (VIN) divided by the peak-to-peak oscillator voltage, VOSC.
FIGURE 7. VOLTAGE-MODE BUCK CONVERTER COMPENSATION DESIGN
Modulator Break Frequency Equations
1 f LC = ----------------------------2 L O C O 1 f ESR = ---------------------------------------2 ( ESR ) ( C O )
The compensation network consists of the error amplifier (internal to the ISL6406, ISL6426) and the impedance networks ZIN and ZFB.The goal of the compensation network is to provide a closed-loop transfer function with the highest 0dB crossing frequency (f 0dB ) and adequate phase margin. Phase margin is the difference between the closed loop phase at f 0dB and 180 degrees. The equations below relate the compensation network's poles, zeros and gain to the components (R1, R2, R3, C1, C2 and C3) in Figure 7. Use these guidelines for locating the poles and zeros of the compensation network: 1. Pick gain (R2/R1) for desired converter bandwidth. 2. Place first zero below filter's double pole (~75% FLC). 3. Place second zero at filter's double pole. 4. Place first pole at the ESR zero. 5. Place second pole at half the switching frequency. 6. Check gain against error amplifier's open-loop gain. 7. Estimate phase margin--repeat if necessary.
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ISL6406, ISL6426
When using the fixed 1.8V output ISL6426 option, the internal resistor values are R1 = 3.5k and R2 = 2.8k, where R1 is connected from VOUT to FB and R2 is connected from FB to GND. the capacitor. A conservative approach is presented in the following equation.
I BIAS + I GATE C PUMP = -------------------------------------- ( 1.5 ) V CC ( f S )
Compensation Break Frequency Equations
Figure 8 shows an asymptotic plot of the DC-DC converter's gain vs frequency. The actual Modulator Gain has a high gain peak due to the high Q factor of the output filter and is not shown in Figure 8. Using the above guidelines should give a Compensation Gain similar to the curve plotted. The open loop error amplifier gain bounds the compensation gain. Check the compensation gain at FP2 with the capabilities of the error amplifier. The Closed Loop Gain is constructed on the graph of Figure 8 by adding the Modulator Gain (in dB) to the Compensation Gain (in dB). This is equivalent to multiplying the modulator transfer function to the compensation transfer function and plotting the gain. The compensation gain uses external impedance networks ZFB and ZIN to provide a stable, high bandwidth (BW) overall loop. A stable control loop has a gain crossing with -20dB/decade slope and a phase margin greater than 45 degrees. Include worst-case component variations when determining phase margin.
FZ1 100 80 60 GAIN (dB) 40 20 0 -20 -40 -60 FZ2 FP1 FP2 OPEN LOOP ERROR AMP GAIN
Output Capacitor Selection
An output capacitor is required to filter the output and supply the load transient current. The filtering requirements are a function of the switching frequency and the ripple current. The load transient requirements are a function of the slew rate (di/dt) and the magnitude of the transient load current. These requirements are generally met with a mix of capacitors and careful layout. Modern digital ICs can produce high transient load slew rates. High-frequency capacitors initially supply the transient and slow the current load rate seen by the bulk capacitors. The bulk filter capacitor values are generally determined by the ESR (Effective Series Resistance) and voltage rating requirements rather than actual capacitance requirements. High-frequency decoupling capacitors should be placed as close to the power pins of the load as physically possible. Be careful not to add inductance in the circuit board wiring that could cancel the usefulness of these low inductance components. Consult with the manufacturer of the load on specific decoupling requirements. Use only specialized low-ESR capacitors intended for switching-regulator applications for the bulk capacitors. The bulk capacitor's ESR will determine the output ripple voltage and the initial voltage drop after a high slew-rate transient. An aluminum electrolytic capacitor's ESR value is related to the case size with lower ESR available in larger case sizes. However, the Equivalent Series Inductance (ESL) of these capacitors increases with case size and can reduce the usefulness of the capacitor to high slew-rate transient loading. Unfortunately, ESL is not a specified parameter. Work with your capacitor supplier and measure the capacitor's impedance with frequency to select a suitable component. In most cases, multiple electrolytic capacitors of small case size perform better than a single large case capacitor.
V IN 20 log --------------- V OSC
COMPENSATION GAIN
R2 20 log ------- R1
MODULATOR GAIN 10 100 FLC 1K FESR 10K 100K 1M LOOP GAIN
10M
FREQUENCY (Hz)
FIGURE 8. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
Output Inductor Selection
The output inductor is selected to meet the output voltage ripple requirements and minimize the converter's response time to the load transient. The inductor value determines the converter's ripple current and the ripple voltage is a function of the ripple current. The ripple voltage and current are approximated by the following equations:
I = VIN - VOUT fs x L x VOUT VIN VOUT = I x ESR
Component Selection Guidelines
Charge Pump Capacitor Selection
A capacitor across pins CT1 and CT2 is required to create the proper bias voltage for the ISL6406, ISL6426 when operating the IC from 3.3V. Selecting the proper capacitance value is important so that the bias current draw and the current required by the MOSFET gates do not overburden
Increasing the value of inductance reduces the ripple current and voltage. However, the large inductance values reduce the converter's response time to a load transient.
12
ISL6406, ISL6426
One of the parameters limiting the converter's response to a load transient is the time required to change the inductor current. Given a sufficiently fast control loop design, the ISL6406, ISL6426 will provide either 0% or 100% duty cycle in response to a load transient. The response time is the time required to slew the inductor current from an initial current value to the transient current level. During this interval the difference between the inductor current and the transient current level must be supplied by the output capacitor. Minimizing the response time can minimize the output capacitance required. The response time to a transient is different for the application of load and the removal of load. The following equations give the approximate response time interval for application and removal of a transient load:
tRISE = L x ITRAN VIN - VOUT tFALL = L x ITRAN VOUT
Some capacitor series available from reputable manufacturers are surge current tested.
MOSFET Selection/Considerations
The ISL6406, ISL6426 requires two N-Channel power MOSFETs. These should be selected based upon rDS(ON) , gate supply requirements, and thermal management requirements. In high-current applications, the MOSFET power dissipation, package selection and heatsink are the dominant design factors. The power dissipation includes two loss components; conduction loss and switching loss. The conduction losses are the largest component of power dissipation for both the upper and the lower MOSFETs. These losses are distributed between the two MOSFETs according to duty factor. The switching losses seen when sourcing current will be different from the switching losses seen when sinking current. When sourcing current, the upper MOSFET realizes most of the switching losses. The lower switch realizes most of the switching losses when the converter is sinking current (see equations on next page). These equations assume linear voltage-current transitions and do not adequately model power loss due the reverse-recovery of the upper and lower MOSFET's body diode. The gate-charge losses are dissipated by the ISL6406, ISL6426 and don't heat the MOSFETs. However, large gatecharge increases the switching interval, tSW which increases the MOSFET switching losses. Ensure that both MOSFETs are within their maximum junction temperature at high ambient temperature by calculating the temperature rise according to package thermal-resistance specifications. A separate heatsink may be necessary depending upon MOSFET power, package type, ambient temperature and air flow. Losses while Sourcing current
2 1 P UPPER = Io x r DS ( ON ) x D + -- Io x V IN x t SW x f s 2 2xr PLOWER = Io DS(ON) x (1 - D)
where: ITRAN is the transient load current step, tRISE is the response time to the application of load, and tFALL is the response time to the removal of load. The worst case response time can be either at the application or removal of load. Be sure to check both of these equations at the minimum and maximum output levels for the worst case response time.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage overshoot across the MOSFETs. Use small ceramic capacitors for high frequency decoupling and bulk capacitors to supply the current needed each time Q1 turns on. Place the small ceramic capacitors physically close to the MOSFETs and between the drain of Q1 and the source of Q2 . The important parameters for the bulk input capacitor are the voltage rating and the RMS current rating. For reliable operation, select the bulk capacitor with voltage and current ratings above the maximum input voltage and largest RMS current required by the circuit. The capacitor voltage rating should be at least 1.25 times greater than the maximum input voltage and a voltage rating of 1.5 times is a conservative guideline. The RMS current rating requirement for the input capacitor of a buck regulator is approximately 1/2 the DC load current. The maximum RMS current required by the regulator may be closely approximated through the following equation:
I RMS = V OUT V IN - V OUT V OUT 2 2 1 ------------- x I OUT + ----- x ---------------------------- x ------------- V IN V IN 12 L x f s MAX
Losses while Sinking current
PUPPER = Io2 x rDS(ON) x D
2 1 P LOWER = Io x r DS ( ON ) x ( 1 - D ) + -- Io x V IN x t SW x f s 2
Where: D is the duty cycle = VOUT / VIN , tSW is the combined switch ON and OFF time, and fs is the switching frequency.
MAX
For a through hole design, several electrolytic capacitors may be needed. For surface mount designs, solid tantalum capacitors can be used, but caution must be exercised with regard to the capacitor surge current rating. These capacitors must be capable of handling the surge-current at power-up. 13
Given the reduced available gate bias voltage (5V), logiclevel or sub-logic-level transistors should be used for both NMOSFETs. Caution should be exercised with devices exhibiting very low VGS(ON) characteristics. The shootthrough protection present aboard the ISL6406, ISL6426 may be circumvented by these MOSFETs if they have large parasitic impedances and/or capacitances that would inhibit the gate of the MOSFET from being discharged below its
ISL6406, ISL6426
threshold level before the complementary MOSFET is turned on. Typical gate charge values for MOSFETs considered in these types of applications range from 20 to 100nC. Since the voltage drop across QLOWER is negligible, VBOOT1 is simply VCPVOUT - VD. A schottky diode is recommended to minimize the voltage drop across the bootstrap capacitor during the on-time of the upper MOSFET. Initial calculations with VBOOT2 no less than 4V will quickly help narrow the bootstrap capacitor range. For example, consider an upper MOSFET is chosen with a maximum gate charge, Qg, of 100nC. Limiting the voltage drop across the bootstrap capacitor to 1V results in a value of no less than 0.1F. The tolerance of the ceramic capacitor should also be considered when selecting the final bootstrap capacitance value. A fast recovery diode is recommended when selecting a bootstrap diode to reduce the impact of reverse recovery charge loss. Otherwise, the recovery charge, QRR, would have to be added to the gate charge of the MOSFET and taken into consideration when calculating the minimum bootstrap capacitance.
Bootstrap Component Selection
External bootstrap components, a diode and capacitor, are required to provide sufficient gate enhancement to the upper MOSFET. The internal MOSFET gate driver is supplied by the external bootstrap circuitry as shown in Figure 9. The boot capacitor, CBOOT, develops a floating supply voltage referenced to the PHASE pin. This supply is refreshed each cycle, when DBOOT conducts, to a voltage of CPVOUT less the boot diode drop, VD, plus the voltage rise across QLOWER.
CPVOUT DBOOT ISL6406 ISL6426 BOOT CBOOT UGATE PHASE QUPPER NOTE: VG-S VCC -VD QLOWER NOTE: VG-S VCC GND + VD VIN
+
LGATE
FIGURE 9. UPPER GATE DRIVE BOOTSTRAP
Just after the PWM switching cycle begins and the charge transfer from the bootstrap capacitor to the gate capacitance is complete, the voltage on the bootstrap capacitor is at its lowest point during the switching cycle. The charge lost on the bootstrap capacitor will be equal to the charge transferred to the equivalent gate-source capacitance of the upper MOSFET as shown:
Q GATE = C BOOT x ( V BOOT1 - V BOOT2 )
where QGATE is the maximum total gate charge of the upper MOSFET, CBOOT is the bootstrap capacitance, VBOOT1 is the bootstrap voltage immediately before turn-on, and VBOOT2 is the bootstrap voltage immediately after turn-on. The bootstrap capacitor begins its refresh cycle when the gate drive begins to turn-off the upper MOSFET. A refresh cycle ends when the upper MOSFET is turned on again, which varies depending on the switching frequency and duty cycle. The minimum bootstrap capacitance can be calculated by rearranging the previous equation and solving for CBOOT.
Q GATE C BOOT = ---------------------------------------------------V BOOT1 - V
BOOT2
14
ISL6406, ISL6426 ISL6406, ISL6426 DC-DC Converter Application Circuit
The circuit below shows the device as it is configured on the ISL6406, ISL6426 evaluation board. Detailed information on the circuit, including a complete Bill-of-Materials and circuit board description, can be found in Application Note AN1031.
3.3V
P1 C1A-B
C3 13 5 VCC CT1 CT2 RT CPVOUT 3 D1 CPGND 1 GND UGATE 10 VOUT SYNC/EN COMP 11 PHASE ISL6406 ISL6426 BOOT 15 C7 16 14 C8A-C Q1 R7 C5 OCSET U1 4 TP3 R1 C2
GND
P2
TP1
C4 R6
6 7 12
C6
R8
L1
2.5V @ 5A P3
P5
8
LGATE FB 9
2
C10 R3 R2 C11 R5 R4 C9 P4 P6 JP1 GND GND
NOTE: Remove R3, R4, C9, and R5 from the board and close JP1 for ISL6426 evaluation.
15
ISL6406, ISL6426 Small Outline Plastic Packages (SOIC)
N INDEX AREA H E -B1 2 3 SEATING PLANE -AD -CA h x 45o 0.25(0.010) M BM
M16.15 (JEDEC MS-012-AC ISSUE C) 16 LEAD NARROW BODY SMALL OUTLINE PLASTIC PACKAGE
INCHES SYMBOL A A1 B MIN 0.053 0.004 0.014 0.007 0.386 0.150 MAX 0.069 0.010 0.019 0.010 0.394 0.157 MILLIMETERS MIN 1.35 0.10 0.35 0.19 9.80 3.80 MAX 1.75 0.25 0.49 0.25 10.00 4.00 NOTES 9 3 4 5 6 7 8o Rev. 1 02/02
L
C D E e H h
C
0.050 BSC 0.228 0.010 0.016 16 0o 8o 0.244 0.020 0.050
1.27 BSC 5.80 0.25 0.40 16 0o 6.20 0.50 1.27
e
B 0.25(0.010) M C AM BS

A1 0.10(0.004)
L N
NOTES: 1. Symbols are defined in the "MO Series Symbol List" in Section 2.2 of Publication Number 95. 2. Dimensioning and tolerancing per ANSI Y14.5M-1982. 3. Dimension "D" does not include mold flash, protrusions or gate burrs. Mold flash, protrusion and gate burrs shall not exceed 0.15mm (0.006 inch) per side. 4. Dimension "E" does not include interlead flash or protrusions. Interlead flash and protrusions shall not exceed 0.25mm (0.010 inch) per side. 5. The chamfer on the body is optional. If it is not present, a visual index feature must be located within the crosshatched area. 6. "L" is the length of terminal for soldering to a substrate. 7. "N" is the number of terminal positions. 8. Terminal numbers are shown for reference only. 9. The lead width "B", as measured 0.36mm (0.014 inch) or greater above the seating plane, shall not exceed a maximum value of 0.61mm (0.024 inch) 10. Controlling dimension: MILLIMETER. Converted inch dimensions are not necessarily exact.
16
ISL6406, ISL6426 Thin Shrink Small Outline Plastic Packages (TSSOP)
N INDEX AREA E E1 -B1 2 3 L 0.05(0.002) -AD -CSEATING PLANE A 0.25 0.010 GAUGE PLANE 0.25(0.010) M BM
M16.173
16 LEAD THIN SHRINK SMALL OUTLINE PLASTIC PACKAGE INCHES SYMBOL A A1 A2 b c D MIN 0.002 0.033 0.0075 0.0035 0.193 0.169 MAX 0.043 0.006 0.037 0.012 0.008 0.201 0.177 MILLIMETERS MIN 0.05 0.85 0.19 0.09 4.90 4.30 MAX 1.10 0.15 0.95 0.30 0.20 5.10 4.50 NOTES 9 3 4 6 7 8o Rev. 1 2/02
A1 0.10(0.004) A2 c
E1 e E L N
e
b 0.10(0.004) M C AM BS
0.026 BSC 0.246 0.020 16 0o 8o 0.256 0.028
0.65 BSC 6.25 0.50 16 0o 6.50 0.70
NOTES: 1. These package dimensions are within allowable dimensions of JEDEC MO-153-AB, Issue E. 2. Dimensioning and tolerancing per ANSI Y14.5M-1982. 3. Dimension "D" does not include mold flash, protrusions or gate burrs. Mold flash, protrusion and gate burrs shall not exceed 0.15mm (0.006 inch) per side. 4. Dimension "E1" does not include interlead flash or protrusions. Interlead flash and protrusions shall not exceed 0.15mm (0.006 inch) per side. 5. The chamfer on the body is optional. If it is not present, a visual index feature must be located within the crosshatched area. 6. "L" is the length of terminal for soldering to a substrate. 7. "N" is the number of terminal positions. 8. Terminal numbers are shown for reference only. 9. Dimension "b" does not include dambar protrusion. Allowable dambar protrusion shall be 0.08mm (0.003 inch) total in excess of "b" dimension at maximum material condition. Minimum space between protrusion and adjacent lead is 0.07mm (0.0027 inch). 10. Controlling dimension: MILLIMETER. Converted inch dimensions are not necessarily exact. (Angles in degrees)
17
ISL6406, ISL6426 Quad Flat No-Lead Plastic Package (QFN) Micro Lead Frame Plastic Package (MLFP)
L16.5x5B
16 LEAD QUAD FLAT NO-LEAD PLASTIC PACKAGE (COMPLIANT TO JEDEC MO-220VHHB ISSUE C) MILLIMETERS SYMBOL A A1 A2 A3 b D D1 D2 E E1 E2 e k L L1 N Nd Ne P 0.25 0.35 2.95 2.95 0.28 MIN 0.80 NOMINAL 0.90 0.20 REF 0.33 5.00 BSC 4.75 BSC 3.10 5.00 BSC 4.75 BSC 3.10 0.80 BSC 0.60 16 4 4 0.60 12 0.75 0.15 3.25 3.25 0.40 MAX 1.00 0.05 1.00 NOTES 9 9 5, 8 9 7, 8 9 7, 8 8 10 2 3 3 9 9 Rev. 1 10/02 NOTES: 1. Dimensioning and tolerancing conform to ASME Y14.5-1994. 2. N is the number of terminals. 3. Nd and Ne refer to the number of terminals on each D and E. 4. All dimensions are in millimeters. Angles are in degrees. 5. Dimension b applies to the metallized terminal and is measured between 0.15mm and 0.30mm from the terminal tip. 6. The configuration of the pin #1 identifier is optional, but must be located within the zone indicated. The pin #1 identifier may be either a mold or mark feature. 7. Dimensions D2 and E2 are for the exposed pads which provide improved electrical and thermal performance. 8. Nominal dimensions are provided to assist with PCB Land Pattern Design efforts, see Intersil Technical Brief TB389. 9. Features and dimensions A2, A3, D1, E1, P & are present when Anvil singulation method is used and not present for saw singulation. 10. Depending on the method of lead termination at the edge of the package, a maximum 0.15mm pull back (L1) maybe present. L minus L1 to be equal to or greater than 0.3mm.
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