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L6997
STEP DOWN CONTROLLER FOR LOW VOLTAGE OPERATIONS
FEATURES s FROM 3V TO 5.5V VCC RANGE. s MINIMUM OUTPUT VOLTAGE AS LOW AS 0.6V. s 1V TO 28V INPUT VOLTAGE RANGE. s CONSTANT ON TIME TOPOLOGY ALLOWS. OPERATION WITH VERYLOW AND HIGH DUTY CYCLES. s VERY FAST LOAD TRANSIENTS. s 0.6V, 1% VREF. s SELECTABLE SINKING MODE. s LOSSLESS CURRENT LIMIT, AVAILABLE ALSO IN SINKING MODE s REMOTE SENSING. s OVP,UVP LATCHED PROTECTIONS. s 600A TYP QUIESCENT CURRENT. s POWER GOOD AND OVP SIGNALS. s PULSE SKIPPING AT LIGTH LOADS. s 94% EFFICIENCY FROM 3.3V TO 2.5V. APPLICATIONS s NETWORKING. s DC/DC MODULES. s DISTRIBUTED POWER. s MOBILE APPLICATIONS. s CHIP SET, CPU, DSP AND MEMORIES SUPPLY. MINIMUM COMPONENT COUNT APPLICATION
3.3V
Rin2 Rin1
TSSOP20 ORDERING NUMBERS: L6997D L6997DTR
DESCRIPTION The device is a high efficient solution for networking dc/dc modules and mobile application compatible with 3.3V bus and 5V bus. It's able to regulate an output voltage as low as 0.6V. The constant on time topology assures fast load transient response. The embedded voltage feed-forward provides nearly constant switching frequency operation. An integrator can be introduced in the control loop to reduce the static output voltage error. The remote sensing improves the static and dynamic regulation, recovering the wires voltage drop. Pulse skipping technique reduces power consumption at light loads. Drivers current capability allows output currents in excess of 20A.
Cin VCC VDR OSC BOOT HGATE HS Cboot L PHASE PGOOD OVP LGATE LS DS Ro2 Ro1 Cout Dboot
0.6V
L6997
ILIM Rilim
PGND
GND GNDSENSE
SS Css
VSENSE INT VFB
SHDN
Vref Cvref
April 2003
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ABSOLUTE MAXIMUM RATINGS
Symbol VCC VDR VCC to GND VDR to GND HGATE and BOOT, to PHASE HGATE and BOOT, to PGND VPHASE PHASE LGATE to PGND ILIM, VFB, VSENSE, NOSKIP, SHDN, PGOOD, OVP, VREF, INT, GNDSENSE to GND Ptot Tstg Power dissipation at Tamb = 25C Storage temperature range Parameter Value -0.3 to 6 -0.3 to 6 -0.3 to 6 -0.3 to 36 -0.3-to 30 -0.3 to VDR+0.3 -0.3 to VCC+0.3 1 -40 to 150 Unit V V V V V V V W C
THERMAL DATA
Symbol Rth j-amb Tj Parameter Thermal Resistance Junction to Ambient Junction operating temperature range Value 125 -40 to 125 Unit C/W C
PIN CONNECTION (Top View)
NOSKIP GNDSENSE INT VSENSE VCC GND VREF VFB OSC SS
1 2 3 4 5 6 7 8 9 10
TSSOP20
20 19 18 17 16 15 14 13 12 11
BOOT HGATE PHASE VDR LGATE PGND PGOOD OVP SHDN ILIM
PIN FUNCTION
N 1 2 3 4 5 6 7 Name NOSKIP GNDSE NSE INT VSENS E VCC GND VREF Remote ground sensing pin Integrator output. Short this pin to VFB pin and connect it via a capacitor to VOUT to insert the integrator in the control loop. If the integrator is not used, short this pin to VREF. This pin must be connected to the remote output voltage to detect overvoltage and undervoltage conditions and to provide integrator feedback input. IC Supply Voltage. Signal ground 0.6V voltage reference. Connect max. a 10nF ceramic capacitor between this pin and ground. This pin is capable to source or sink up to 250uA Description Connect to VCC to force continuous conduction mode and sink mode.
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PIN FUNCTION (continued)
N 8 9 10 11 12 13 14 15 16 17 18 19 20 Name VFB OSC SS ILIM SHDN OVP PGOOD PGND LGATE VDR PHASE HGATE BOOT Description PWM comparator feedback input. Short this pin to INT pin when using the integrator function, or to VSENSE pin without integrator. Connect this pin to the input voltage through a voltage divider in order to provide the feedforward function. It cannot be left floating. Soft start pin. A 5A constant current charges an external capacitor which value sets the softstart time. An external resistor connected between this pin and GND sets the current limit threshold. Shutdown. When connected to GND the device and the drivers are OFF. It cannot be left floating. Open drain output. During the over voltage condition it is pulled up by an external resistor. Open drain output. During the soft start and in case of output voltage fault it is low. It is pulled up by external resistor. Low Side driver ground. Low Side driver output. Low Side driver supply. Return path of the High Side driver. High side MOSFETS driver output. Bootstrap capacitor pin. High Side driver is supplied through this pin.
ELECTRICAL CHARACTERISTICS (VCC = VDR = 3.3V; Tamb = 0C to 85C unless otherwise specified)
Symbol SUPPLY SECTION Vin VCC, VDR VCC Turn-onvoltage Turn-off voltage Hysteresis IqVDR IqVcc Quiescent Current Drivers Device Quiescent current VFB > VREF VFB > VREF Input voltage range Vout=Vref Fsw=110Khz Iout=1A 1 3 2.86 2.75 90 7 400 20 600 28 5.5 2.97 2.9 V V V V mV A A V 0.6 SHDN to GND SHDN to GND 1 5 15 V A A A mV A mV A Parameter Test Condition Min. Typ. Max. Unit
SHUTDOWN SECTION SHDN Device On Device Off ISHVDR ISHVCC Drivers shutdown current Devices shutdown current 1.2
SOFT START SECTION ISS Soft Start current Active Soft start and voltage CURRENT LIMIT AND ZERO CURRENT COMPARATOR ILIM input bias current Zero Crossing Comparator offset Phase-gnd DKILIM Current limit factor RILIM = 2K to 200K 4.6 -2 1.6 1.8 5 5.4 2 2 VSS = 0.4V 4 300 400 6 500
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ELECTRICAL CHARACTERISTICS (continued) (VCC = VDR = 3.3V; Tamb = 0C to 85C unless otherwise specified)
Symbol ON TIME Ton On time duration VREF=VSENSE OSC=125mV VREF=VSENSE OSC=250mV VREF=VSENSE OSC=500mV OFF TIME TOFFMIN Minimum off time KOSC/TOFFMIN VOLTAGE REFERENCE VREF Voltage Accuracy 0A < IREF < 100A 0.594 0.6 0.606 V OSC=250mV 0.3 600 0.33 ns 720 370 210 800 420 240 880 470 270 ns ns ns Parameter Test Condition Min. Typ. Max. Unit
PWM COMPARATOR Input voltage offset IFB Input Bias Current -2 20 +2 mV nA
INTEGRATOR INT INT Over Voltage Clamp Under Voltage Clamp Integrator Input Offset Voltage VSENSE-VREF IVSENSE Input Bias Current VSENSE = VCC VSENSE = GND 0.62 0.45 -4 20 0.75 0.55 0.88 0.65 -4 V V mV nA
GATE DRIVERS High side rise time High side fall time Low side rise time Low side fall time PGOOD UVP/OVP PROTECTIONS OVP UVP PGOOD PGOOD VPGOOD Over voltage threshold Under voltage threshold Upper threshold (VSENSE-VREF) Lower threshold (VSENSE-VREF) VSENSE rising VSENSE falling ISink=2mA with respect to VREF 118 67 110 85 121 70 112 88 0.2 124 73 116 91 0.4 % % % % V VDR=3.3V; C=7nF HGATE - PHASE from 1 to 3V VDR=3.3V; C=14nF LGATE from 1 to 3V 50 50 50 50 90 100 90 90 ns ns ns ns
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Vcc
SHDN + VSENSE 1.12 VREF undervoltage comparator VSENSE 0.6 VREF IC enable pgood comparators + VSENSE VCC VSENSE 0.925 VREF HGATE BOOT V(LGATE)<0.5V comp + LS and HS anti-cross-conduction comparators 1.075 VREF soft-start control S R +
PGOOD
OVP
overvoltage comparator VCC GND
SS
5 uA
ILIM
power management
V IN
R Q V(PHASE)<0.2V comp PHASE level shifter HS driver
Figure 1. Functional & Block Diagram
positive current limit comparator
Toff min delay S
V OUT
PHASE Ton min one-shot Q LS driver
+
VDR
0.05 R Ton one-shot
+
LGATE
VREF S VSENSE OSC
+
PGND
FB
-
pwm comparator
Ton= Kosc V(VSENSE)/V(OSC)
S Q
INT
+ -
Ton
one-shot
Gm
HS control
+ R
VREF
SENSEGND no-skip mode no-skip mode + + -
Ton= Kosc V(VSENSE)/V(OSC)
OSC
bandgap PHASE zero-cross comparator +
PHASE
negative current limit comparator
1.236V
VREF
0.6V
1.416
0.05
ILIM
Reference chain
NOSKIP
LS control
VSENSE
OSC
V IN
VREF VSENSE
L6997
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1 DEVICE DESCRIPTION
1.1 Constant On Time PWM topology Figure 2. Loop block schematic diagram
Vin R1 One-shot generator OSC FFSR RQ HGATE Vref S Q LS DS
R2
Vsense
HS
Vout
+ PWM comparator FB
LGATE
R4
R3
The device implements a Constant On Time control scheme, where the Ton is the high side MOSFET on time duration forced by the one-shot generator. The on time is directly proportional to VSENSE pin voltage and inverse to OSC pin voltage as in Eq1: Eq 1 V SENSE T O N = K O SC --------------------- + V
O SC
where KOSC = 250ns and is the internal propagation delay time (typ. 70ns). The system imposes in steady state a minimum on time corresponding to VOSC = 1V. In fact if the VOSC voltage increases above 1V the corresponding Ton will not decrease. Connecting the OSC pin to a voltage partition from VIN to GND, it allows a steady-state switching frequency FSW independent of VIN. It results: Eq 2 where Eq 3 R2 V OS C O SC = -------------- = -------------------V IN R 2 + R1 O SC 1 V OUT 1 f SW = -------------- ---------- = -------------- -------------- O SC = f SW K O SC OUT V IN T O N OUT K O SC
Eq 4
V FB R4 OUT = -------------- = -------------------V OUT R3 + R4
The above equations allow setting the frequency divider ratio OSC once output voltage has been set; note that such equations hold only if VOSC<1V. Further the Eq2 shows how the system has a switching frequency ideally independent from the input voltage. The delay introduces a light dependence from VIN. A minimum off-time constrain of about 500ns is introduced in order to assure the boot capacitor charge and to limit the switching fre6/23
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quency after a load transient as well as to mask PWM comparator output against noise and spikes. The system has not an internal clock, because this is a hysteretic controller, so the turn on pulse will start if three conditions are met contemporarily: the FB pin voltage is lower than the reference voltage, the minimum off time is passed and the current limit comparator is not triggered (i.e. the inductor current is below the current limit value). The voltage on the OSC pin must range between 50mV and 1V to ensure the system linearity. 1.2 Closing the loop The loop is closed connecting the output voltage (or the output divider middle point) to the FB pin. The FB pin is linked internally to the comparator negative pin and the positive pin is connected to the reference voltage (0.6V Typ.) as in Figure 2. When the FB goes lower than the reference voltage, the PWM comparator output goes high and sets the flip-flop output, turning on the high side MOSFET. This condition is latched to avoid noise spike. After the on-time (calculated as described in the previous section) the system resets the flip-flop and then turns off the high side MOSFET and turns on the low side MOSFET. Internally the device has more complex logic than a flip-flop to manage the transition in correct way. For more details refers to the Figure 1. The voltage drop along ground and supply metals connecting output capacitor to the load is a source of DC error. Further the system regulates the output voltage valley value not the average, as in the Figure 3 is shown. So the voltage ripple on the output capacitor is a source of DC static error (as the PCB traces). To compensate the DC errors, an integrator network must be introduced in the control loop, by connecting the output voltage to the INT pin through a capacitor and the FB pin to the INT pin directly as in Figure 4. The internal integrator amplifier with the external capacitor CINT1 introduces a DC pole in the control loop. CINT1 also provides an AC path for output ripple. Figure 3. Valley regulation
Vout
DC Error Offset

Vref
Time
The integrator amplifier generates a current, proportional to the DC errors, that increases the output capacitance voltage in order to compensate the total static errors. A voltage clamper within the device forces INT pin voltage ranges from VREF-50mV, VREF+150mV. This is useful to avoid or smooth output voltage overshoot during a load transient. Also, this means that the integrator is capable of recovering output error due to ripple when its peakto-peak amplitude is less than 150mV in steady state. In case of the ripple amplitude is larger than 150mV, a capacitor CINT2 can be connected between INT pin and ground to reduce ripple amplitude at INT pin, otherwise the integrator can operate out of its linear range. Choose CINT1 according to the following equation: Eq 5 g INT OUT C INT1 = -----------------------------2 Fu
where GINT=50 s is the integrator transconductance, OUT is the output divider ratio given from Eq4 and FU is the close loop bandwidth. This equation also holds if CINT2 is connected between INT pin and ground. CINT2 is given by:
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Eq 6 C INT 2 V OUT --------------- = -----------------C INT 1 V INT
Where VOUT is the output ripple and VINT is the ripple wanted at the INT pin (100mV typ). Figure 4. Integrator loop block diagram
Vin
R1 One-shot generator OSC FFSR RQ From Vsense Vref S Q LS FB DS LOAD HGATE PCB TRACES
R2
HS
Vout
+ PWM comparator Vref
LGATE
INT Cint2
+ +
Vsense
Gndsense
Integrator amplifier Cint1
Respect to a traditional PWM controller, that has an internal oscillator setting the switching frequency, in a hysteretic system the frequency can change with some parameters (input voltage, output current). In L6997 is implemented the voltage feed-forward circuit that allows constant switching frequency during steady-sate operation with the input voltage variation. There are many factors affecting switching frequency accuracy in steady-state operation. Some of these are internal as dead times, which depend on high side MOSFET driver. Others related to the external components as high side MOSFET gate charge and gate resistance, voltage drops on supply and ground rails, low side and high side RDSON and inductor parasitic resistance. During a positive load transient, (the output current increases), the converter switches at its maximum frequency (the period is TON+TOFFmin) to recover the output voltage drop. During a negative load transient, (the output current decreases), the device stops to switch (high side MOSFET remains off). 1.3 Transition from PWM to PFM/PSK To achieve high efficiency at light load conditions, PFM mode is provided. The PFM mode differs from the PWM mode essentially for the off section; the on section is the same. In PFM after a turn-on cycle the system turnson the low side MOSFET, until the inductor current reaches the zero A value, when the zero-crossing comparator turns off the low side MOSFET. In this way the energy stored in the output capacitor will not flow to ground, through the low side MOSFET, but it will flow to the load. In PWM mode, after a turn on cycle, the system keeps the low side MOSFET on until the next turn-on cycle, so the energy stored in the output capacitor will flow through the low side MOSFET to ground. The PFM mode is naturally implemented in hysteretic controller, in fact in PFM mode the system reads the output voltage with a comparator and then turns on the high side MOSFET when the output voltage goes down a reference value. The device works in discontinuous mode at light load and in continuous mode at high load. The transition from PFM to PWM occurs when load current is around half the inductor current ripple. This threshold value depends on VIN, L, and VOUT. Note that the higher the in-
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ductor value is, the smaller the threshold is. On the other hand, the bigger the inductor value is, the slower the transient response is. In PFM mode the frequency changes, with the output current changing, more than in PWM mode; in fact if the output current increase, the output voltage decreases more quickly; so the successive turn-on arrives before, increasing the switching frequency. The PFM waveforms may appear more noisy and asynchronous than normal operation, but this is normal behaviour mainly due to the very low load. If the PFM is not compatible with the application it can be disabled connecting to VCC the NOSKIP pin. 1.4 Softstart If the supply voltages are already applied, the SHDN pin gives the start-up. The system starts with the high side MOSFET and the low side MOSFET off (high impedance mode). After the SHDN pin is turned on the SS pin voltage begins to increase and the system starts to switch. The softstart is realized by gradually increasing the current limit threshold to avoid output overvoltage. The active soft start range for the VSS voltage (where the output current limit increase linearly) starts from 0.6V to 1V. In this range an internal current source (5A Typ) charges the capacitor on the SS pin; the reference current (for the current limit comparator) forced through ILIM pin is proportional to SS pin voltage and it saturates at 5A (Typ.) when SS voltage is close to 1V and the maximum current limit is active. Output protections OVP & UVP are disabled until the SS pin voltage reaches 1V (see figure 5). Once the SS pin voltage reaches the 1V value, the voltage on SS pin doesn't impact the system operation anymore. If the SHDN pin is turned on before the supplies, the correct start-up sequence is the following: first turnon the power section and after the logic section (VCC pin). Figure 5. Soft -Start Diagram
Vss
4.1V 1V
Soft-start active range 0.6V
Ilim current
5A
Time
Maximum current limit
Time
Because the system implements the soft start controlling the inductor current, the soft start capacitor selection is function of the output capacitance, the current limit and the soft start active range (VSS). In order to select the softstart capacitor it must be imposed that the output voltage reaches the final value before the soft start voltage reaches the under voltage value (1V). In other words the output voltage charging time has to be lower than the uvp time. The UVP time is given by: Eq 7 V uv p T uv p ( C SS ) = ------------ C SS Iss
In order to calculate the output volatge chargin time it should be calculated, before, the output volatrge function versus time. This function can be calculated from the inductor current function; the inductor current function can
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be supposed linear function of the time. Eq 8 ( R ilim /R ds on K C I SS t ) I L ( t,C SS ) = ------------------------------------------------------------------( V SS C SS )
so the output voltage is given by: Eq 9 Q ( t,C SS ) ( R ilim /R ds on K C I S S t ) V o ut ( t,C S S ) = ------------------------ = ---------------------------------------------------------------------( C ou t V SS C SS 2 ) C ou t
2
calling Vout as the Vout final value, the output charging time can be estimated as: Eq 10 ( V ou t C o ut V S S C SS 2 ) 0.5 I out ( C SS ) = --------------------------------------------------------------------------( R ilim /R dso n K C I SS )
the minimum CSS value is given imposing this condition: Eq 11 Tout =Tuvp
1.5 Current limit The current limit comparator senses the inductor current through the low side MOSFET RDSON drop and compares this value with the ILIM pin voltage value. While the current is above the current limit value, the control inhibits the one-shot start. To properly set the current limit threshold, it should be noted that this is a valley current limit. Average current depends on the inductor value, VIN VOUT and switching frequency. The average output current in current limit is given by: Eq 12 I OUT I = I m ax valley + ---2
CL
Thus, to set the current threshold, choose RILIM according to the following equation: Eq 13
m ax valley
R IL im = ---------------- K ILIM Rd s o n
In current limit the system keeps the current constant until the output voltage meets the undervolatge threshold. The negative valley current limit, for the sink mode, is set automatically at the same value of the positive valley current limit. The average negative current limit differs from the positive average current limit by the ripple current; this difference is due to the valley control technique. The system accuracy is function of the exactness of the resistance connected to ILIM pin and the low side MOSFET RDSON accuracy. Moreover the voltage on ILIM pin must range between 10mV and 1V to ensure the system linearity. Figure 6. Current limit schematic
To inductor LS
RILIM
PGN D
Current Comparator
PHASE
5A
Positive and negative current limit
To logic
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1.6 Protection and fault Sensing VSENSE pin voltage performs output protection. The nature of the fault (that is, latched OV or latched UV) is given by the PGOOD and OVP pins. If the output voltage is between the 89% (typ.) and 110% (typ) of the regulated value, PGOOD is high. If a hard overvoltage or an undervoltage occurs, the device is latched: low side MOSFET and, high side MOSFET are turned off and PGOOD goes low. In case the system detects an overvoltage the OVP pin goes high. To recover the functionality the device must be shut down and restarted thought the SHDN pin, or the supply has to be removed, and restart with the correct sequence. These features are useful to protect against short-circuit (UV fault) as well as high side MOSFET short (OV fault). 1.7 Drivers The integrated high-current drivers allow using different size of power MOSFET, maintaining fast switching transition. The driver for the high side MOSFET uses the BOOT pin for supply and PHASE pin for return (floating driver). The driver for the low side MOSFET uses the VDR pin for the supply and PGND pin for the return. The main feature is the adaptive anti-cross-conduction protection, which prevents from both high side and low side MOSFET to be on at the same time, avoiding a high current to flow from VIN to GND. When high side MOSFET is turned off the voltage on the pin PHASE begins to fall; the low side MOSFET is turned on only when the voltage on PHASE pin reaches 250mV. When low side is turned off, high side remains off until LGATE pin voltage reaches 500mV. This is important since the driver can work properly with a large range of external power MOSFETS. The current necessary to switch the external MOSFETS flows through the device, and it is proportional to the MOSFET gate charge and the switching frequency. So the power dissipation of the device is function of the external power MOSFET gate charge and switching frequency. Eq 14 P drive r = V cc Q g TO T F SW
The maximum gate charge values for the low side and high side are given from: Eq 15 f SW 0 Q MAX HS = ------------ 75n C f SW f SW0 Q MAX LS = ------------ 125 nC f SW
Eq 16
Where fSW0 = 500Khz. The equations above are valid for TJ = 150C. If the system temperature is lower the QG can be higher. For the Low Side driver the max output gate charge meets another limit due to the internal traces degradation; in this case the maximum value is QMAXLS = 125nC. The low side driver has been designed to have a low resistance pull-down transistor, around 0.5 ohms. This prevents the voltage on LGATE pin raises during the fast rise-time of the pin PHASE, due to the Miller effect. Because the driver voltage can be very low it should be considered also the ULTRA LOW VOLTAGE MOSFET. This kind of MOSFET has very low threshold voltage, so the overdrive voltage can be enough to ensure correct transition and low enough RDSON.
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2 APPLICATION INFORMATION
2.1 5A Demo board description The demo board shows the device operation in this condition: VIN from 3.3V to 5V, IOUT=5A VOUT=1.25V. The evaluation board let use the system with 2 different voltages (VCC the supply for the IC and VIN the power input for the conversion) so replacing the input capacitors the power input voltage could be also 28V. When instead the input voltage (VIN) is equal to the VCC it should be better joining them with a 10 resistor in order to filter the device input voltage. On the topside demo there are two different jumpers: one jumper, near the OVP and POWER GOOD test points, is used to shut down the device; when the jumper is present the device is in SHUTDOWN mode, to run the device remove the jumper. The other jumper, near the VREF test point, is used to set the PFM/ PSK mode. When the jumper is present, at light load, the system will go in PFM mode; if there is not the jumper, at light load, the system will remain in PWM mode. In the demo bottom side there are two others different jumpers. They are used to set or remove the INTEGRATOR configuration. When the jumpers named with INT label are closed AND the jumpers named with the NOINT label are open the integrator configuration is set. Sometimes the integrator configuration needs a low frequency filter the to reduce the noise interaction. In this case instead close the INT jumpers put there a resistor and after a capacitor to ground (as in the schematic diagram); the pole value is around 500Khz but it should be higher enough than the switching frequency (ten times). On the opposite when the jumpers named with the NOINT are closed and the jumpers named with INT are open the NON INTEGRATOR configuration is selected. Refer to the Table 1 and 2 for the jumpers connection. Figure 7. Demoboard Schematic Diagram
Vcc
R7
C8
R6
J1
C7,C13 VIin
R4 OSC BOOT C4 HGATE R5 TP1 PGOOD OVP TP2 ILIM R8 C12 LGATE Q2 D2 R1 R10 PHASE Q1 D1 L1 VCC VDR
C11 C10
GNDin
VOUT
R3 C14,C15 R2 PGND GND NOSKIP VSENSE GNDSENSE GNDOUT
L6997
SS C9 R9 INT VFB SHDN SD VREF C6
NOINT INT C1 NOINT TP3 C5
C3
INT C2 NS Cn Rn
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2.2 Jumper Connection Table 1. Jumper connection with integrator
Component C1 C2 INT NOINT Connection Mounted Mounted * Close Open
* This component is not necessary, depends from the output ESR capacitor. See the integrator section.
Table 2. Jumper connection without integrator
Component C1 C2 INT NOINT Connection Not mounted Not Mounted Open Close
2.3 DEMOBOARD LAYOUT Real dimensions: 4,7 cm X 2,7 cm (1.85 inch X 1. 063 inch) Figure 8. Top side components placement Figure 10. Top side layout
Figure 9. Bottom side Jumpers distribution
Figure 11. Bottom side layout
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Table 3. PCB Layout guidelines
Goal Low radiation and low magnetic coupling with the adjacent circuitry. Suggestion 1) Small switching current loop areas. (For example placing CIN, High Side and Low side MOSFETS, Shottky diode as close as possible). 2) Controller placed as close as possible to the power MOSFET. 3) Group the gate drive component (Boot cap and diode together near the IC. Keep power traces and load connections short and wide. Phase pin and PGND pin must be made with Kelvin connection and as close as possible to the Low Side MOSFETS. 1) Put the feedback component (like output divider, integrator network, etc) as close as possible to the IC. 2) The feedback traces must be parallel and as close as possible. Moreover they must be routed as far as possible from the switching current loops..
Don't penalty the efficiency. Ensure high accuracy in the current sense system. Reduce the noise effect on IC.
Table 4. Component list The component list is shared in two sections: the first for the general-purpose component, the second for power section: GENERAL-PURPOSE SECTION
Part name RESISTOR R1, R5, R9, R10 R2 R3 R4 R6 R7 R8 CAPACITOR C1 C2 C3 C4 C5 C6 C9 C10 C11 C8, C12 DIODE D1 BAR18 330pF N.M. 1nF 100nF 1F 10nF 10nF 100nF 100nF 47pF 0603 0603 0603 0603 Tantalum 0603 0603 0603 0603 0603 Softstart capacitor First integrator capacitor Second integrator capacitor 470k 0 33k 1k 1.1k 0603 0603 0603 0603 0603 0603 0603 Current limit resistor Input resistor divider (To set switching frequency) Pull-up resistor Output resistor divider (To set output voltage) Value Dimension Notes
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POWER SECTION
INPUT CAPACITORS C7, C13 OUTPUT CAPACITORS C14, C15 INDUCTOR 220F 2R5TPE220M POSCAP DO3316P-272HC COILCRAFT STMicroelectronics STMicroelectronics Double mosfet in sigle package 3 47F ECJ4XF0J476Z PANASONIC
POWER MOS Q1,Q2 DIODE D2 STPS340U
Notes: 1. N.M.=Not Mounted 2. The demoboard with this component list is set to give: VOUT = 1.25V, F SW = 270kHz with an input voltage around VIN = VCC = 3.3V-5V and with the integrator feature. 3. The diode efficiency impact is very low; it is not a necessary component. 4. All capacitors are intended ceramic type otherwise specified.
STS5DNF20V
2.4 EFFICIENCY CURVES Source mode VIN = 3.3V VOUT = 1.25V FSW = 270kHz Figure 12. Efficiency vs output current
Eff [%]
100,0 90,0 80,0 70,0 60,0 50,0 40,0 30,0 20,0 10,0 0,0 0,0 1,0 2,0 3,0 4,0 5,0 6,0 PWM mode
L1
2.7 H
PFM mode
Current [A]
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3 STEP BY STEP DESIGN
VIN = 3.3V, 10% VOUT = 1.25V IOUT = 5A FSW = 270kHz 3.1 Input capacitor. A pulsed current (with zero average value) flows through the input capacitor of a buck converter. The AC component of this current is quite high and dissipates a considerable amount of power on the ESR of the capacitor: Eq 17
2 Vin ( Vin - Vo ut ) P CIN = ESR CIN Iout ----------------------------------------------2 Vin
The RMS current, which the capacitor must provide, is given by: Eq 18 Icin rm s =
2 2 Iout ( 1 - ) + ----- ( I L ) 12
Neglecting the last term, the equation reduces to: Eq 19 Icin rm s = Io ut ( 1 - )
which maximum value corresponds to to = 1/2. ICINRMS, has a maximum equal to = 1/2 (@ VIN = 2xVOUT, that is, 50% duty cycle). The input, therefore, should be selected for a RMS ripple current rating as high as half the respective maximum output current. Electrolytic capacitors are the most used cause are the cheapest ones and are available with a wide range of RMS current ratings. The only drawback is that, considering a requested ripple current rating, they are physically larger than other capacitors. Very good tantalum capacitors are coming available, with very low ESR and small size. The only problem is that they occasionally can burn if subjected to very high current during the charge. So, it is better avoid this type of capacitors for the input filter of the device. In fact, they can be subjected to high surge current when connected to the power supply. If available for the requested value and voltage rating, the ceramic capacitors have usually a higher RMS current rating for a given physical dimension (due to the very low ESR). The drawback is the quite high cost. Possible solutions:
10F 22F 47F 33F C34Y5U1E106ZTE12 TOKIN JMK325BJ226MM TAIYO-YUDEN ECJ4XF0J476Z PANASONIC C3225X5R0J476M TDK
With our parameter from the equation 3 it is found: Icinrms = 2.42A 3.2 Inductor To define the inductor, it is necessary to determine firstly the inductance value. Its minimum value is given by: Eq 20 V o ( Vin m ax - V o ) Lmin -------------------------------------------------------------F SW I out RF Vin m ax
where RF is given from I/IOUT (basically it is around 30%).
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With our parameters: Lmin 2H The saturation current is around 5A 3.3 Output capacitor and ripple voltage The output capacitor is chosen by the output voltage static precision and also dynamic precision. The static precision regards the output voltage ripple value rated the output voltage in steady state at the end the ESR value; while the dynamic precision regards the load step positive and negative load transient. If the static precision is around 1% for the 1.25V output voltage, the output precision is 12.5mV. To determine the ESR value from the output precision is necessary to calculate the ripple current: Eq 21 Vin - Vo Vo I = ---------------------- -------- T s w L Vin
One can consider a switching frequency around 270kHz. From the Eq. above the ripple current is around 1.25A. So the ESR is given from: RMS current in output capacitor is given by: Eq 22 V ripp le 25mV ESR = -------------------- = --------------- = 20m I 1.25 ---2
The dynamic specifications are sometimes more relaxed than the static requirements, so one can consider the ESR value around 20m enough. To allow the device control loop to properly work, output capacitor ESR zero must be at least ten times smaller than switching frequency. Low ESR tantalum capacitors, which ESR zero is close to ten kHz, are suitable for output filtering. Output capacitor value COUT and its ESR, ESRCOUT, should be large enough and small enough, respectively, to keep output voltage within the accuracy range during a load transient, and to give the device a minimum signal to noise ratio. The current ripple flows through the output capacitors, so the should be calculated also to sustain this ripple: the RMS current value is given by Eq. 18. Eq 23 1 Icout rms = ---------- I L 23
But this is usually a negligible constrain. Possible solutions:
330F 220F EEFUE0D331R PANASONIC 2R5TPE220M POSCAP
3.4 MOSFET's and Schottky Diodes Since a 3.3V bus powers the gate drivers of the device, the use ultra low level MOSFET is highly recommended, especially for high current applications. The MOSFET breakdown voltage VBRDSS must be greater than VINMAX with a certain margin, so the selection will address 20V or 30V devices (depends on applications). The RDSON can be selected once the allowable power dissipation has been established. By selecting identical Power MOSFET as the main switch and the synchronous rectifier, the total power they dissipate does not depend on the duty cycle. Thus, if PON is this power loss (few percent of the rated output power), the required RDSON (@ 25 C) can be derived from:
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Eq 24 PO N RDS O N = -----------------------------------------------2 Io ut ( 1 + T )
is the temperature coefficient of RDSON (typically, = 510-3 C-1 for these low-voltage classes) and T the admitted temperature rise. It is worth noticing, however, that generally the lower RDSON, the higher is the gate charge QG, which leads to a higher gate drive consumption. In fact, each switching cycle, a charge QG moves from the input source to ground, resulting in an equivalent drive current: Eq 25 Iq = Qg F SW
The SCHOTTKY diode to be placed in parallel to the synchronous rectifier must have a reverse voltage VRRM greater than VINMAX; for low current application the SCHOTTKY is not necessary to increase the efficiency. In order to use less space than possible, a double MOSFET in a single package is chosen: STS5DNF20V 3.5 Output voltage setting The first step is choosing the output divider to set the output voltage. To select this value there isn't a criteria, but a low divider network value (around 100) decries the efficiency at low current; instead a high value divider network (100K) increase the noise effects. A network divider values from 1K to 10K is right. We chose: R3 = 1K R2 = 1.1K The device output voltage is adjustable by connecting a voltage divider from output to VSENSE pin. Minimum output voltage is VOUT=VREF=0.6V. Once output divider and frequency divider have been designed as to obtain the required output voltage and switching frequency, the following equation gives the smallest input voltage, which allows L6997 to regulate (which corresponds to TOFF=TOFFMIN): OS C 1 < 1 - -------------- -------------------------------------------- OUT K O SC ------------------------- MAX T OF F,M IN

Eq 26
3.6 Voltage Feedforward From the equations 1,2 3 choosing the switching frequency around 270kHz it can be selected the input divider. For example: R3 = 470K R4 = 8.5K 3.7 Current limit resistor From the equation 8 it can be set the valley current limit considering the STS5DNF20V Ultra logic Level Mosfet with a current around 5A: R8 = 120K 3.8 Integrator capacitor Let it be FU = 15kHz, VOUT = 1.25V. Since VREF = 0.6V, from equation 2, of the device description, it follows OUT = 0.348 and, from equation 5 it follows C = 250pF. The output ripple is around 22mV, so the system doesn't need the second integrator capacitor.
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3.9 Soft start capacitor Considering the soft start equations: CSS = 150pF The equations are valid without load. When an active load is present the equations result more complex; further some active loads have unexpected effect, as higher current than the expected one during the soft start, can change the start up time. In this case the capacitor value can be selected on the application; anyway the Eq11 gives an idea about the CSS value. 3.10 Sink mode Figure 13. Efficiency vs output current
Eff [%]
100,0 90,0 80,0 70,0 60,0 50,0 40,0 30,0 20,0 10,0 0,0 0,0 1,0 2,0 3,0 4,0 5,0
Current [A]
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4 TYPICAL OPERATING CHARACTERISTICS Figure 16. Normal functionality in PWM mode.
Figure 14. Load transient response from 0A to 5A..
Ch1-> Inductor current Ch2-> Phase Node Ch3-> Output voltage
Ch1-> Inductor current Ch2-> Phase Node Ch3-> Output voltage
Figure 15. Normal functionality in SINK mode..
Figure 17. Normal functionality in PFM mode.
Ch1-> Inductor current Ch2-> Phase Node Ch3-> Output voltage
Ch1-> Inductor current Ch2-> Phase Node Ch3-> Output voltage
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Figure 18. Start up waveform with 0A load. Figure 19. Start up waveform with 5A load..
Ch1-> Inductor current Ch2-> Soft start Voltage Ch3-> Output voltage
Ch1-> Inductor current Ch2-> Soft start Voltage Ch3-> Output voltage
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mm DIM. MIN. A A1 A2 b c D (1) E E1 (1) e L L1 k aaa
Note:
inch MAX. 1.20 MIN. TYP. MAX. 0.047 0.002 0.031 0.007 0.004 0.252 0.244 0.170 0.256 0.252 0.173 0.026 0.750 0.018 0.024 0.039 0 (min.) 8 (max.) 0.100 0.004 0.030 0.039 0.006 0.041 0.012 0.008 0.260 0.260 0.177
TYP.
OUTLINE AND MECHANICAL DATA
0.050 0.800 0.190 0.090 6.400 6.200 4.300 6.500 6.400 4.400 0.650 0.450 0.600 1.000 1.000
0.150 1.050 0.300 0.200 6.600 6.600 4.500
TSSOP20
Thin Shrink Small Outline Package
1. D and E1 does not include mold flash or protrusions. Mold flash or potrusions shall not exceed 0.15mm (.006inch) per side.
0087225 (Jedec MO-153-AC)
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Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics (c) 2003 STMicroelectronics - All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan -Malaysia - Malta - Morocco Singapore - Spain - Sweden - Switzerland - United Kingdom - United States. http://www.st.com
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