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RT9202
Single Synchronous Buck PWM DC-DC Controller
General Description
The RT9202 is a single power supply PWM DC-DC converter controller designed to drive N-channel MOSFET in a synchronous buck topology. The IC integrates the control, output adjustment, monitoring and protection functions in a small 8-pin package. The RT9202 uses a low gain voltage mode PWM control for simple application design. An internal 0.8V reference allows the output voltage to be precisely regulated to low voltage requirement. A fixed 300kHz oscillator reduces the component size for saving board space. The RT9202 features over current protection, over voltage protection, and under voltage lock-out. The output current is monitored by sensing the voltage drop across the MOSFET's RDS(ON), which eliminates the need for a current sensing resistor.
Features
Operate from 5V 0.8V Internal Reference Drive Two N-channel MOSFET Voltage Mode PWM Control Fast Transient Response Fixed 300kHz Oscillator Frequency Full 0~100% Duty Cycle Internal Soft Start Adaptive Non-overlapping Gate Driver Over-current Monitor Uses MOSFET RDS(ON) Over-voltage Protection Uses Low-side MOSFET
Pin Configurations
Part Number RT9202CS (Plastic SOP-8) Pin Configurations
TOP VIEW
BOOT 1 UGATE 2 GND 3 LGATE 4
8 7 6 5
PHASE OCSET FB VCC
Applications
Motherboard Power Regulation for Computers Subsystems Power Supplies Cable Modems, Set Top Box, and DSL Modems DSP and Core Communications processor Supplies Memory Power Supplies Personal Computer Peripherals Industrial Power Supplies 5V-Input DC-DC Regulators Low Voltage Distributed Power Supplies
Ordering Information
RT9202 Package type S : SOP-8 Operating temperature range C: Commercial standard
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RT9202
Typical Application Circuit
R1 20K R4 10 D1 8 7 SHDN H: shutdown Q2 2N7002 6 5 C4 1F PHASE OCSET FB VCC BOOT UGATE GND LGATE R3 120 1 2 3 4 VOUT 2.5V + C3 L2 5H ML C2 0.1F MU C5 1F + C1 470F MA732 5V
RT9202
1000F R2 255 C6 10nF
Fig.1 RT9202 powered from 5V only
R4 12V R1 20K 10 5V
H: shutdown Q1 2N7002
8 7 6 5 C4 1F
PHASE OCSET FB VCC
BOOT 1 UGATE GND 2 3
C2 1F MU VOUT 2.5V + C3 L1 5H 1000F R2 250 C6 10nF ML C5 1F + C1 470F
SHDN
RT9202
LGATE 4 R3 120
Fig.2 RT9202 powered from 12V
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RT9202
MU
+ COUT 1000F
D
L 5H
G
S
CIN1 1F + CIN2 470F
CVCC 1F
GND VCC BOOT
CBOOT
ML
0.1F
RT9202
D G S
GND Return
Layout Placement
Layout Notes 1. Put CIN1 & CIN2 to be near the MU drain and ML source nodes. 2. Put RT9202 to be near the COUT 3. Put CBOOT as close as to BOOT pin 4. Put CVCC as close as to VCC pin
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RT9202
Function Block Diagram
VCC Power on Reset
6.0V Regulator Bias
BOOT
40A 0.8V Reference Soft Start OC _ _ 1V
OVP
OCSET
+
UGATE
+ 0.5V 0.8V Error 35dB + _ +
UVP
Control Logic
PWM
PHASE VCC LGATE
+
GND
300kHz Oscillator
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4
_
_
FB
Error Amp
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RT9202
Absolute Maximum Ratings
Supply Voltage VCC 7V BOOT & UGATE to GND 15V Input, Output or I/O Voltage GND-0.3V ~ 7V Power Dissipation, PD @ TA = 25C SOP-8 0.625W Package Thermal Resistance SOP-8, JA 160C/W Ambient Temperature Range 0C ~ +70C Junction Temperature Range -40C ~ +125C Storage Temperature Range -65C ~ +150C Lead Temperature (Soldering, 10 sec.) 260C CAUTION: Stresses beyond the ratings specified 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.
Electrical Characteristics
(VCC = 5V, TA = 25C, Unless otherwise specified.) Parameter Symbol Test Conditions Min Typ Max Units VCC Supply Current / Regulated Voltage Nominal Supply Current Regulated Voltage from BOOT Power-On Reset Rising VCC Threshold VCC Threshold Hysteresis Rising VOCSET Threshold Reference Reference Voltage Oscillator Free Running Frequency Ramp Amplitude Error Amplifier DC gain PWM Controller Gate Driver Upper Drive Source Upper Drive Sink Lower Drive Source Lower Drive Sink RUGATE RUGATE RLGATE RLGATE BOOT= 12V BOOT-VUGATE = 1V VUGATE = 1V VCC - VLGATE = 1V, VLGATE = 1V ----7 5 4 2 11 7.5 6 4 32 35 38 dB VOSC 250 -300 1.75 350 -KHz VP-P 0.784 0.8 0.816 V VOCSET = 4.5V VOCSET1 = 4.5V 3.85 0.3 0.8 4.1 0.5 1.25 4.35 0.7 2.0 V V V ICC VCC UGATE, LGATE open VBOOT = 12V -5 3 6 6 7 mA V
To be continued
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Parameter Protection FB Over-Voltage Trip FB Under-Voltage Trip OCSET Current Source Soft-Start Interval IOCSET FB Rising FB Falling VOCSET= 4.5V 1.0 -35 1 1.1 0.5 40 2 -0.6 45 4 V V A mS Symbol Test Conditions Min Typ Max Units
Functional Pin Description
BOOT (Pin 1) 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 Nchannel MOSFET when operating at a single 5V power supply. This pin also could be powered from ATX 12V, in this situation, a internal 6.0V regulator will supply to VCC pin for internal voltage bias. UGATE (Pin 2) Connect UGATE pin to the PWM converter's upper MOSFET gate. This pin provides the gate drive for the upper MOSFET. GND (Pin 3) Signal and power ground for the IC. All voltage levels are measured with respect to this pin. LGATE (Pin 4) Connect LGATE to the PWM converter's lower MOSFET gate. This pin provides the gate drive for the lower MOSFET. VCC (Pin 5) This is the main bias supply for the RT9202. This pin also provides the gate bias charge for the lower MOSFETs gate. The voltage at this pin monitored for power-on reset (POR) purpose. This pin is also the internal 6.0V regulator output powered from BOOT pin when BOOT pin is directly powered from ATX 12V. FB (Pin 6) This pin is connected to the PWM converter's output divider. This pin also connects to internal PWM error amplifier inverting input and protection monitor. OCSET (Pin 7) Connect a resistor from this pin to the drain of the respective upper MOSFET. This resistor, an internal 40A current source, and the upper MOSFET onresistance set the converter over-current trip point. An over-current trip cycles the soft-start function. The voltage at this pin is monitored for power-on reset (POR) purpose and pulling this pin low with an open drain device will shut down the IC.
IPEAK =
IOCSET x ROCSET RDS(ON)
PHASE (Pin 8) This pin is used to monitor the voltage drop across the upper MOSFET for over-current protection.
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RT9202
Typical Operating Charateristics
Dead Time
VCC = 5V UGATE UGATE
Dead Time
VCC = 5V
LGATE
LGATE
Time
Time
Power On
VCC = 5V VOUT = 2.2V VCC
Power Off
VCC = 5V VOUT = 2.2V
VCC
VOUT
VOUT
Time
Time
Load Transient
UGATE
Load Transient
UGATE VCC = 5V VOUT = 2.2V COUT = 3000F VOUT
VOUT VCC = 5V VOUT = 2.2V COUT = 3000F
Time
Time
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RT9202
Bootstrap Wave Form
VCC = 5V; VOUT = 2.2V VCC = 5V VOUT = 2.2V VOUT LGATE
Short Hiccup
UGATE
PHASE UGATE Time Time
Reference vs. Temperature
0.803 0.802 0.801
55 50 45
IOCSET vs. Temperature
Reference (V)
0.800 0.799 0.798 0.797 0.796 -50 0
IOCSET ( A)
50 100 150
40 35 30 25 20
Temperature ( C)
-40
-10
20
50
80
110
140
Temperature ( C)
POR (Rising/Falling) vs. Temperature
4.3 4.2 4.1
Oscillator Frequency vs. Temperature
315 310 305
Rising
Frequency (kHz)
300 295 290 285 280 275
POR (V)
4.0 3.9 3.8
Falling
3.7 3.6 -50 0 50 100 150
270
Temperature ( C)
-50
0
50
100
150
Temperature ( C)
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RT9202
Functional Description
The RT9202 operates at either single 5V power supply with a bootstrap UGATE driver or 5V/12V dual-power supply form the ATX SMPS. The dualpower supply is recommended for high current application, the RT9202 can deliver higher gate driving current while operating with ATX SMPS based on dual-power supply. The Bootstrap Operation In a single power supply system, the UGATE driver of RT9202 is powered by an external bootstrap circuit, as the Fig.1. The boot capacitor, CBOOT, generates a floating reference at the PHASE pin. Typically a 0.1F CBOOT is enough for most of MOSFETs used with the RT9202. The voltage drop between BOOT and PHASE is refreshed to a voltage of VCC - diode drop (VD) while the low side MOSFET turning on.
R1
VCC
6.0V Regulator
BOOT C1 1F
R1 10
12V 5V +
UGATE
VCC C2 1F RT9202 LGATE
Fig.2 Dual Power Supply Operation Power On Reset The Power-On Reset (POR) monitors the supply voltage (normal +5V) at the VCC pin and the input voltage at the OCSET pin. The VCC POR level is 4.1V with 0.5V hysteresis and the normal level at OCSET pin is 1.5V (see over-current protection). The POR function initiates soft-start operation after all supply voltages exceed their POR thresholds. Soft Start A built-in soft-start is used to prevent surge current from power supply input during power on. The softstart voltage is controlled by an internal digital counter. It clamps the ramping of reference voltage at the input of error amplifier and the pulse-width of the output driver slowly. The typical soft-start duration is 2mS. Over-Current Protection The over current protection (OCP) function of the RT9202 is triggered when the voltage across the RDS(ON) of upper side MOSFET that developed by drain current exceeds over-current tripping level. An external resistor (ROCSET) programs the over-current tripping level of the PWM converter. As shown on Fig.1, the internal 40A current sink (IOCSET) develops a voltage across ROCSET (VSET) that is referenced to VIN. The DRIVE signal enables the over-current comparator (OC). When the voltage across the upper MOSFET (VDS(ON)) exceeds VSET, the over-current
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C2 1F
VCC
BOOT UGATE PHASE
D1 0.1F +
5V
VCC LGATE
RT9202
Fig.1 Single 5V power Supply Operation Dual Power Operation The RT9202 was designed to regulate a 6.0V at VCC pin automatically when BOOT pin is powered by 12V. In a system with ATX 5V/12V power supply, the RT9202 is ideal for higher current application due to the higher gate driving capability, VUGATE = 7V and VLGATE = 6.0V. A RC (10/1F) filter is also recommended at BOOT pin to prevent the ringing induced from fast power on, as shown in Fig.2.
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comparator trips to set the over-current latch. Both VSET and VDS are referenced to VIN and a small capacitor across ROCSET helps VOCSET tracking the variations of VIN due to MOSFET switching. The overcurrent function will be tripped at a peak inductor current (IPEAK) determined by: The OC trip point varies with MOSFET's RDS(ON) temperature variations. The temperature coefficient of IOCSET is 2500ppm that is used to compensate RDS(ON) temperature variations. To avoid over-current tripping in the normal operating load range, determine the ROCSET resistor value from the equation above with: 1. The maximum RSD(ON) at the highest junction temperature 2. The minimum IOCSET from the characteristics 3. Determine IPEAK for IPEAK > IOUT(MAX) + (I)/2 where I is the output inductor ripple current.
COUNT = 1 COUNT = 2 Internal SS 4V 2V 0V OVERLOAD APPLIED COUNT = 3
INDUCTOR CURRENT
0A T0T1 T2 TIME T3
Fig. 4 Shutdown Pulling low the OCSET pin by a small single transistor can shutdown the RT9202 PWM controller as shown in typical application circuit.
OVER-CURRENT TRIP: VDS > VSET
iD xR DS(ON) > IOCSET x ROCSET
VIN = +5V ROCSET VSET+ iD
OCSET IOCSET 40A DRIVE + _
VCC UGATE
VDS+
OC
PWM
GATE CONTROL
PHASE VPHASE = VIN - VDS VOCSET = VIN - VSET
Fig.3 Under Voltage and Over Voltage Protection The voltage at FB pin is monitored and protected against OC (over current), UV (under voltage), and OV (over voltage). The UV threshold is 0.5V and OVthreshold is 1.0V. Both UV/OV detection have 30S triggered delay. When OC or UV trigged, a hiccup restart sequence will be initialized, as shown in Fig.4. Only 3 times of trigger are allowed to latch off. Hiccup is disabled during soft-start interval.
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RT9202
Applications Information
Inductor Selection The RT9202 was designed for VIN = 5V, step-down application mainly. Fig.5 shows the typical topology and waveforms of step-down converter. The ripple current of inductor can be calculated as follows: ILRIPPLE = (5V - VOUT)/L x TON Because operation frequency is fixed at 300kHz, TON = 3.33 x VOUT/5V The VOUT ripple is VOUT RIPPLE = ILRIPPLE x ESR ESR is output capacitor equivalent series resistor Table 1 shows the ripple voltage of VOUT: VIN = 5V
iL IL Q IL = IO
Q L VL VI D C R VO
C.C.M.
TS
TON VI VL
TOFF
- VO - VO
iQ IQ
iD ID
Fig.5 Table 1 VOUT Inductor 1000F (ESR=53m) 1500F (ESR=33m) 3000F (ESR=21m) 2H 100mV 62mV 40mV 3.3V 5H 40mV 25mV 16mV 2H 110mV 68mV 43mV 2.5V 5H 44mV 28mV 18mV 2H 93mV 58mV 37mV 1.5V 5H 37mV 23mV 15mV
*Refer to Sanyo low ESR series (CE, DX, PX...) The suggested L and C are as follows: 2H with 1500F COUT 5H with 1000F COUT
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RT9202
Input / Output Capacitor High frequency/long life decoupling capacitors should be placed as close to the power pins of the load as physically possible. Be careful not to add inductance to the PCB trace, as it could eliminate the performance from utilizing these low inductance components. Consult with the manufacturer of the load on specific decoupling requirements. The output capacitors are necessary for filtering output and stabilizing the close loop (see the PWM loop stability). For powering advanced, high-speed processors, it is required to meet with the requirement of fast load transient, high frequency capacitors with low ESR/ESL capacitors are recommended. Another concern is high ESR induced ripple may trigger UV or OV protections. PWM Loop Stability The RT9202 is a voltage mode buck controller designed for 5V step-down applications. The gain of error amplifier is fixed at 35dB for simplified design. The output amplitude of ramp oscillator is 1.6V, the loop gain and loop pole/zero are calculated as follows: DC loop gain GA = 35dB x
5 0.8 x 1.6 VOUT 1 LC filter pole PO = x x LC 2
Reference Voltage Because RT9202 use a low 35dB gain error amplifier, shown in Fig. 7. The voltage regulation is dependent on VIN & VOUT setting. The FB reference voltage of 0.8V were trimmed at VIN = 5V & VOUT = 2.5V condition. In a fixed VIN = 5V application, the FB reference voltage vs. VOUT voltage can be calculated as Fig. 8.
I3 56K FB I2 1K REP 0.8V _ RAMP 1.75V _ EA + +
PWM
+ _
Fig. 7
0.82 0.81 FB (V) 0.80 0.79 VIN = 5V
0.78 0.5
1
1.5
2
2.5 VOUT (V)
3
3.5
4
4.5
Fig. 8 Feedback Divider The reference of RT9202 is 0.8V. The output voltage can be set using a resistor based divider as shown in Fig.9. Put the R1 and R2 as close as possible to FB pin and R2 should less than 1 k to avoid noise coupling. The C1 capacitor is a speed-up capacitor for reducing output ripple to meet with the requirement of fast transient load. Typically a 1nF ~ 0.1F is enough for C1.
Error Amp pole PA = 300kHz ESR zero ZO =
1 x x ESR x C 2
The RT9202 Bode plot as shown Fig.6 is stable in most of application conditions.
VOUT = 3.3V COUT = 1500F(33m) L=2H 40 VOUT = 1.5V VOUT = 2.5V 30 VOUT = 3.3V PO = 2.9kHz ZO = 3.2kHz
20 Loop Gain
10
100
1k
10k
100k
1M
Fig. 6
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RT9202
VIN L VOUT + C OUT R1 C1 RT9202 FB R2 < 1K
especially the high-frequency ceramic decoupling capacitors, close to the power switches. Place the output inductor and output capacitors between the MOSFETs and the load. Also locate the PWM controller near by MOSFETs. A multi-layer printed circuit board is recommended. Fig.10 shows the connections of the critical components in the converter. Note that the capacitors CIN and COUT each of them represents numerous physical capacitors. Use a dedicated grounding plane and use vias to ground all critical components to this layer. Apply another solid layer as a power plane and cut this plane into smaller islands of common voltage levels. The power plane should support the input power and output power nodes. Use copper filled polygons on the top and bottom circuit layers for the PHASE node, but it is not necessary to oversize this particular island. Since the PHASE node is subjected to very high dV/dt voltages, the stray capacitance formed between these island and the surrounding circuitry will tend to couple switching noise. Use the remaining printed circuit layers for small signal routing. The PCB traces between the PWM controller and the gate of MOSFET and also the traces connecting source of MOSFETs should be sized to carry 2A peak currents.
Fig. 9 PWM Layout Considerations MOSFETs switch very fast and efficiently. The speed with which the current transitions from one device to another causes voltage spikes across the interconnecting impedances and parasitic circuit elements. The voltage spikes can degrade efficiency and radiate noise, that results in ocer-voltage stress on devices. Careful component placement layout and printed circuit design can minimize the voltage spikes induced in the converter. Consider, as an example, the turn-off transition of the upper MOSFET prior to turn-off, the upper MOSFET was carrying the full load current. During turn-off, current stops flowing in the upper MOSFET and is picked up by the low side MOSFET or Schottky diode. Any inductance in the switched current path generates a large voltage spike during the switching interval. Careful component selections, layout of the critical components, and use shorter and wider PCB traces help in minimizing the magnitude of voltage spikes. There are two sets of critical components in a DC-DC converter using the RT9202. The switching power components are most critical because they switch large amounts of energy, and as such, they tend to generate equally large amounts of noise. The critical small signal components are those connected to sensitive nodes or those supplying critical bypass current. The power components and the PWM controller should be placed firstly. Place the input capacitors,
IQ1
IL VOUT
5V + Q1 IQ2 Q2 GND +
+
LOAD
LGATE UGATE
VCC
GND FB
RT9202
Fig. 10
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RT9202
Package Information
H A M
JB
F
C
D
I
Dimensions In Millimeters Symbol A B C D F H I J M Min 4.801 3.810 1.346 0.330 1.194 0.178 0.102 5.791 0.406 Max 5.004 3.988 1.753 0.508 1.346 0.254 0.254 6.198 1.270
Dimensions In Inches Min 0.189 0.150 0.053 0.013 0.047 0.007 0.004 0.228 0.016 Max 0.197 0.157 0.069 0.020 0.053 0.010 0.010 0.244 0.050
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RT9202
RICHTEK TECHNOLOGY CORP.
Headquarter
5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611
RICHTEK TECHNOLOGY CORP.
Taipei Office (Marketing)
8F-1, No. 137, Lane 235, Paochiao Road, Hsintien City Taipei County, Taiwan, R.O.C. Tel: (8862)89191466 Fax: (8862)89191465 Email: marketing@richtek.com
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