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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-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
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Operate From 5V 0.8V Internal Reference Drive Two N-MOSFETs Voltage Mode PWM Control Fast Transient Response Fixed 300kHz Oscillator Frequency Full 0 to 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 RoHS Compliant and 100% Lead (Pb)-Free
Applications
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Ordering Information
RT9202 Package Type S : SOP-8 Operating Temperature Range P : Pb Free with Commercial Standard G : Green (Halogen Free with Commercial Standard) Note : RichTek Pb-free and Green products are : }RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020. }Suitable for use in SnPb or Pb-free soldering processes. }100%matte tin (Sn) plating.
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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
Pin Configurations
(TOP VIEW)
BOOT UGATE GND LGATE 2 3 4 8 7 6 5 PHASE OCSET FB VCC
SOP-8
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RT9202
Typical Application Circuit
R1 20K R4 10 D1 MA732
5V
C2 8 7 SHND H : shutdown Q1 2N7002 6 5 C4 1uF PHASE BOOT 1 2 3 4 0.1uF VOUT 2.5V
OCSET UGATE FB GND
L2 + 5uH C3 1000uF
MU + C5 1uF C1 470uF
LGATE VCC RT9202 R3 120
ML R2 255 C6 10nF
Figure 1. RT9202 powered from 5V only
R4 R1 20K 10
12V 5V
8 7 SHND H : shutdown Q1 2N7002 5V 6 5 C4 1uF
PHASE
BOOT
1 2 3 4
C2 1uF MU
+
OCSET UGATE FB GND
VOUT 2.5V
+
L1 5uH C3 1000uF
C5 1uF
C1 470uF
LGATE VCC RT9202 R3 120
ML R2 250 C6 10nF
Figure 2. RT9202 powered from 12V and 5V
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RT9202
MU COUT 1000uF GND CBOOT VCC BOOT 0.1uF RT9202 G S GND Return ML D D L 5uH G S
+ +
CVCC 1uF
CIN1 1uF
CIN2 470uF
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
Function Block Diagram
VCC Power on Reset
6.0V Regulation Bias
BOOT
0.8 Reference
Soft Start
40uA
+ OC 1.1V OVP +
OCSET
UGATE
0.5V UVP + 0.8V FB Error + Error Amplifier + PWM -
Control Logic VCC
PHASE
LGATE
GND
300kHz Oscillator
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RT9202
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 N-MOSFET when operating at a single 5V power supply. This pin also could be powered from ATX 12V, in this situation, an 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 MOSFET gate. The voltage at this pin is 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 upper MOSFET. This resistor, an internal 40A current source, and the upper MOSFET on-resistance set the converter over-current trip point. An over-current trip cycles the softstart 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.
I x R OCSET IPEAK = OCSET R DS(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
Absolute Maximum Ratings
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(Note 1) 7V 15V
Supply Input Voltage, VCC ----------------------------------------------------------------------------------------BOOT & UGATE to GND -------------------------------------------------------------------------------------------Input, Output or I/O Voltage ---------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25C SOP-8 ------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 4) SOP-8, JA -------------------------------------------------------------------------------------------------------------Ambient Temperature Range --------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.)--------------------------------------------------------------------------Storage Temperature Range ---------------------------------------------------------------------------------------ESD Susceptibility (Note 2) HBM (Human Body Mode) -----------------------------------------------------------------------------------------MM (Machine Mode) ------------------------------------------------------------------------------------------------(Note 3)
GND - 0.3V to 7V
0.625W 160C/W 0C to +70C 260C -65C to +150C 2kV 200V
Recommended Operating Conditions
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Junction Temperature Range --------------------------------------------------------------------------------------- -40C to +125C
Electrical Characteristics
(VCC = 5V, TA = 25 C, 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 VOCSET = 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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RT9202
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
Note 1. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are for stress ratings. Functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may remain possibility to affect device reliability. Note 2. Devices are ESD sensitive. Handling precaution recommended. Note 3. The device is not guaranteed to function outside its operating conditions. Note 4. JA is measured in the natural convection at T A = 25C on a low effective thermal conductivity test board of JEDEC 51-3 thermal measurement standard.
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RT9202
Typical Operating Characteristic
Dead Time
VCC = 5V UGATE UGATE
Dead Time
VCC = 5V
LGATE
LGATE
Time (50ns/Div)
Time (50ns/Div)
Power On
VCC = 5V VOUT = 2.2V VCC
Power Off
VCC VCC = 5V VOUT = 2.2V
VOUT
VOUT
Time (2.5ms/Div)
Time (50ms/Div)
Load Transient
UGATE
Load Transient
UGATE
VOUT
VCC = 5V VOUT = 2.2V COUT = 3000uF VCC = 5V VOUT = 2.2V COUT = 3000uF VOUT
Time (5us/Div)
Time (5us/Div)
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RT9202
Bootstrap Wave Form Short Circuit Hiccup
VCC = 5V, VOUT = 2.2V UGATE VOUT
LGATE
PHASE UGATE VCC = 5V, VOUT = 2.2V
Time (1us/Div)
Time (5ms/Div)
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 50 100 150
IOCSET (uA)
40 35 30 25 20 -40 -10 20 50 80 110 140
Temperature (C)
Temperature (C)
POR (Rising/Falling) vs. Temperature
4.3 4.2 4.1
Oscillator Frequency vs. Temperature
315 310
Rising
Frequency (kHz) A
305 300 295 290 285 280 275 270
POR (V)
4.0 3.9 3.8 3.7 3.6 -50 0 50 100 150
Falling
-50
0
50
100
150
Temperature (C)
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Temperature (C)
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RT9202
Application Information
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 dual- power 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
LGATE 5V 6.0V Regulation BOOT C1 1uF UGATE R1 10 12V 5V + VCC
VCC
In a single power supply system, the UGATE driver of RT9202 is powered by an external bootstrap circuit, as the Figure 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 C2 1uF D1 5V 0.1uF
+
C2 1uF RT9202
Figure 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
BOOT
UGATE PHASE VCC LGATE
RT9202
Figure 1. Single 5V power Supply Operation Dual Power Operation The RT9202 is 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 Figure 2.
A built-in soft-start is used to prevent surge current from power supply input during power on. The soft-start 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 Figure 3 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 overcurrent comparator trips to set the over-current latch.
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RT9202
Both VSET and VDS are referenced to VIN and a small capacitor across ROCSET helps V OCSET tracking the variations of VIN due to MOSFET switching. The overcurrent function will be tripped at a peak inductor current (IPEAK) determined by :
I x R OCSET IPEAK = OCSET RDS(ON) COUNT = 1 COUNT = 2 COUNT = 3
Internal SS INDUCTOR CURRENT
4V 2V 0V OVERLOAD APPLIED
The OC trip point varies with MOSFET's R DS(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 R SD(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.
OVER-CURRENT TRIP: VDS > VSET iD x RDS(ON) > IOCSET x ROCSET OCSET IOCSET 40uF DRIVE + OC PHASE
GATE CONTROL
0A T0 T1 T2 TIME T3
Figure 4 Shutdown Pulling low the OCSET pin can shutdown the RT9202 PWM controller as shown in typical application circuit. Inductor Selection The RT9202 was designed for VIN = 5V, step-down application mainly. Figure 5 shows the typical topology and waveforms of step-down converter. The ripple current of inductor can be calculated as follows:
VIN = +5V
ROCSET VSET+ VCC UGATE VDS+ iD
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
PWM
VPHASE = VIN - VDS VOCSET = VIN - VSET
Figure 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 OV-threshold is 1.0V. Both UV/OV detection have 30s triggered delay. When OC or UV trigged, a hiccup re-start sequence will be initialized, as shown in Figure 4. Only 3 times of trigger are allowed to latch off. Hiccup is disabled during softstart interval.
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RT9202
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
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 manuf acturer 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.
uQ IL = IO
Q
L VL
VI
D
C
R
VO
C.C.M
TS
TON
TOFF VI - VO
VL - VO iL mIL
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 1.75V, the loop gain and loop pole/zero are calculated as follows :
iQ IQ
DC loop gain GA = 35dB x 5 x 0.8 1.75 VOUT 1 LC filter pole PO = 2 LC Error Amp pole PA = 300kHz
iD ID
ESR zero ZO =
1 2 ESRC
Figure 5
The RT9202 Bode plot as shown Figure 6 is stable in most of application conditions.
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RT9202
Feedback Divider
VOUT = 3.3V COUT = 1500F(33m) L = 2H 40 VOUT = 1.5V VOUT = 2.5V VOUT = 3.3V PO = 2.9kHz ZO = 3.2kHz
30
20 Loop Gain 10
The reference of RT9202 is 0.8V. The output voltage can be set using a resistor based divider as shown in Figure 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 to 0.1F is enough for C1.
VIN
100
1k
10k
100k
1M
L
Figure 6 Reference Voltage Because RT9202 use a low 35dB gain error amplifier, shown in Figure 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 Figure 8.
R2 FB + R1 1K REP 0.8V +
EA
VOUT
+
COUT R1 R2 <1K C1 RT9202 FB
Figure 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 over-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
56K + RAMP 1.75V
PWM
Figure 7
0.82 VIN = 5V 0.81
FB (V)
0.80 0.79 0.78 0.5
1
1.5
2
2.5 VOUT (V)
3
3.5
4
4.5
selections, layout of the critical components, and use shorter and wider PCB traces help in minimizing the magnitude of voltage spikes.
Figure 8
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RT9202
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, 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. Figure 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.
IQ1 IL VOUT Q1 IQ2 Q2 GND
+ + +
5V
LOAD
UGATE
GND LGATE VCC RT9202 FB
Figure 10
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RT9202
Outline Dimension
A
H M
J
B
F
C I D
Dimensions In Millimeters Symbol Min A B C D F H I J M 4.801 3.810 1.346 0.330 1.194 0.170 0.050 5.791 0.400 Max 5.004 3.988 1.753 0.508 1.346 0.254 0.254 6.200 1.270
Dimensions In Inches Min 0.189 0.150 0.053 0.013 0.047 0.007 0.002 0.228 0.016 Max 0.197 0.157 0.069 0.020 0.053 0.010 0.010 0.244 0.050
8-Lead SOP Plastic Package
Richtek Technology Corporation
Headquarter 5F, No. 20, Taiyuen Street, Chupei City Hsinchu, Taiwan, R.O.C. Tel: (8863)5526789 Fax: (8863)5526611
Richtek Technology Corporation
Taipei Office (Marketing) 8F, 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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