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RT9209/A
Synchronous Buck PWM DC-DC with Enable & PGOOD
General Description
The RT9209/A 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 into a small 8-pin package. The RT9209/A uses an internal compensated 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 260kHz/ 400kHz oscillator reduce the component size for saving board area. The RT9209/A future a enable control pin to shutdown PWM switching and a 90% power good flag indicator. The FB pin under voltage detection function monitor the output short circuit which trigger a three time hiccup sequence to latch off the chip function.
Features
Operates at 5V 0.8V Internal Reference Drives Two N-MOSFET Voltage Mode PWM Control Fast Transient Response Fixed 260kHz/400kHz Oscillator Frequency Dynamic 0 to 100% Duty Cycle Internal PWM Loop Compensation Internal Soft-Start Adaptive Non-Overlapping Gate Driver Over-Voltage Protection Uses Lower MOSFET RoHS Compliant and 100% Lead (Pb)-Free
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
RT9209/A Package Type S : SOP-8 Operating Temperature Range P : Pb Free with Commercial Standard G : Green (Halogen Free with Commercial Standard) 400kHz 260kHz
Pin Configurations
(TOP VIEW)
FB 8 2 3 4 7 6 5 SS PGOOD BOOT UGATE
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.
VCC LGATE GND
SOP-8
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RT9209/A
Typical Application Circuit
5V R1 120 R2 250 R3 10K PGOOD 1 2 C3 1uF 3 4 FB VCC LGATE SS PGOOD BOOT 8 7 6 5 C2 0.1uF D1 MU CBOOT 0.1uF A ML L1 VOUT 5uH C4 1000uF C1 470uF
GND UGATE RT9209/A
Figure A. RT9209/A Booted from 5V
5V R1 120 R2 250 R4 10K PGOOD 1 2 C3 1uF 3 4 FB VCC LGATE SS PGOOD BOOT 8 7 6 5 C2 0.1uF A ML MU L1 VOUT 5uH C4 1000uF C1 470uF
GND UGATE RT9209/A
R3 12V 10 C5 1uF
Figure B. RT9209/A Booted from 12V
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RT9209/A
MU COUT 1000uF GND CVCC 1uF CBOOT VCC BOOT 0.1uF RT9209/A GND Return Diode D L 5uH G S
+ +
CIN1 1uF
CIN2 470uF
Layout Placement
Layout Notes 1. Put CIN1 & CIN2 to be near the MU drain and ML source nodes. 2. Put RT9209/A 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 + PGOOD Soft Start 0.72V 0.8 Reference 1V +
OVP
6.5V Regulation
BOOT
PGOOD
SS
UGATE
0.5V +UVP 0.8V FB + SS Error Amplifier + +PWM -
Control Logic VCC LGATE
PWM Loop Compensation GND 260kHz/300kHz Oscillator
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RT9209/A
Functional Pin Description
FB (Pin1) 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. VCC (Pin 2) This is the main bias supply for the RT9209/A. 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 6.5V regulator output powered from BOOT pin when BOOT pin is directly powered from ATX 12V. LGATE (Pin 3) Connect LGATE to the PWM converter's lower MOSFET gate. This pin provides the gate drive for the lower MOSFET. GND (Pin 4) Signal and power ground for the IC. All voltage levels are measured with respect to this pin. UGATE (Pin 5) Connect UGATE pin to the PWM converter's upper MOSFET gate. This pin provides the gate drive for the upper MOSFET. BOOT (Pin 6) 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 when operating at a single 5V power supply. This pin also could be powered from ATX 12V, in this situation, a internal 6.5V regulator will supply to VCC pin for internal voltage bias. PGOOD (Pin 7) PGOOD is an open collector output used to indicate the status of the PWM converter output voltage. This pin is pulled low when the FB is not over 90% of the reference voltage. SS (Pin 8) Connect a capacitor from this pin to ground. This capacitor, along with an internal 22A current source, sets the softstart internal of the synchronous PWM converter.
Absolute Maximum Ratings
Supply Input Voltage, VCC ------------------------------------------------------------------------------------------- 7V BOOT & UGATE to GND --------------------------------------------------------------------------------------------- 19V Input, Output or I/O Voltage ----------------------------------------------------------------------------------------- GND-0.3V to 7V Power Dissipation, PD @ TA = 25C SOP-8 -------------------------------------------------------------------------------------------------------------------- 0.625W Package Thermal Resistance SOP-8, JA -------------------------------------------------------------------------------------------------------------- 160 C/W Ambient Temperature Range ---------------------------------------------------------------------------------------- 0C to +70C Junction Temperature Range ---------------------------------------------------------------------------------------- 0C to +125C Storage Temperature Range ---------------------------------------------------------------------------------------- -65C to +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.
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RT9209/A
Electrical Characteristics
(VCC = 5V, TA = 25C, Unless otherwise specified.)
Parameter VCC Supply Current Nominal Supply Current Power-On Reset Rising VCC Threshold VCC Threshold Hysteresis 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 Protection FB Over-Voltage Trip FB Under-Voltage Trip Power Good Threshold Power Good Hysteresis Power Good Sink Capability SS Source Current RT9209 RT9209A
Symbol
Test Conditions
Min
Typ
Max
Units
ICC
UGATE, LGATE open
--
3
6
mA
3.7 0.3
4.1 0.5
4.5 0.7
V V
0.784
0.8
0.816
V
210 350 VOSC --
260 400 1.75
310 450 --
kHz VP-P
33
35
40
dB
RUGATE RUGATE RLGATE RLGATE
BOOT= 12V BOOT-VUGATE = 1V VUGATE = 1V VCC - VLGATE = 1V, VLGATE = 1V
-----
7 7 7 3
12 7.5 6 4

FB Rising FB Falling FB pin Rising
--86 --
1 0.6 90 2
--95 -0.4
V V % % V A
IPGOOD = 1mA
-12 20
32
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RT9209/A
Typical Operating Characteristics
Dead Time
Booted from 5V UGATE UGATE LGATE
Dead Time
Booted from 5V
LGATE
IOUT = 0A
IOUT = 0A
Time (50ns/Div)
Time (50ns/Div)
Dead Time
Booted from 12V UGATE
Dead Time
Booted from 12V
UGATE LGATE
LGATE IOUT = 0A IOUT = 0A
Time (50ns/Div)
Time (50ns/Div)
Load Transient
UGATE
Load Transient
UGATE
VOUT VCC = 5V VOUT = 2.5V COUT = 1000uF IOUT = 10A to 0A VCC = 5V VOUT = 2.5V COUT = 1000uF IOUT = 0A to 10A VOUT
IOUT
IOUT
Time (10us/Div)
Time (10us/Div)
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RT9209/A
Power On
Booted from 5V IOUT = 10A CSS = 0.1uF
Power Off
Booted from 5V IOUT = 10A VCC VOUT VCC
VOUT
Time (2.5ms/Div)
Time (10ms/Div)
Power On
Booted from 12V IOUT = 10A VCC
Power Off
Booted from 12V IOUT = 10A
VCC
VOUT VOUT
Time (2.5ms/Div)
Time (50ms/Div)
Boostrap Waveform
Booted from 5V
Boostrap Waveform
UGATE UGATE
LGATE
LGATE
A node
Booted from 12V, A node
Time (1us/Div)
Time (1us/Div)
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RT9209/A
Short Hiccup
Booted from 5V Booted from 12V
Short Hiccup
UGATE
UGATE
VOUT
VOUT
Time (500ms/Div)
Time (25ms/Div)
VCC vs. VSS
IOUT = 10A CSS = 0.1uF
PGOOD vs. VOUT
VCC PGOOD
VSS VOUT
Time (5ms/Div)
Time (25ms/Div)
Reference vs. Temperature
0.808 0.806 0.804
400 410
Oscillator Frequency vs. Temperature
Reference (V)
0.802 0.8 0.798 0.796 0.794 0.792 0.79 0.788 -35 -15 5 25 45 65 85 105 125
Frequency (kHz)
390 380 370 360
RT9209A
350 -35 -15 5 25 45 65 85 105 125
Temperature (C)
Temperature (C)
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RT9209/A
POR(Rising/Falling) vs. Temperature
4.2 4.1 4
Rising
POR (V)
3.9 3.8 3.7
Falling
3.6 3.5 -35 -15 5 25 45 65 85 105 125
Temperature (C)
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RT9209/A
Application Information
The RT9209/A operates at either single 5V power supply with a bootstrap UGATE driver or a 5V/12V dual-power supply form the ATX SMPS. The dual- power supply is recommended for high current applications, the RT9209/A can deliver higher gate driving current while operating with ATX SMPS based on a dual-power supply. The Bootstrap Operation In a single power supply system, the UGATE driver of RT9209/A is powered by an external bootstrap circuit, as shown in the Figure1. The boot capacitor, C BOOT , generates a floating reference. Typically a 0.1F CBOOT is enough for most of MOSFETs used with the RT9209/A. The voltage drop between BOOT and A node is refreshed to a voltage of VCC - diode drop (VD) while the lower MOSFET turning on.
C2 1uF LGATE 5V VCC 6.5V Regulator BOOT C1 1uF UGATE R1 10 + 12V 5V
A VCC
RT9209/A
Figure 2. Dual Power Supply Operation Power On Reset The Power-On Reset (POR) monitors the supply voltage (normal +5V) at the VCC pin. The VCC POR level is set to 4.1V with 0.5V hysteresis. 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 soft-start voltage is controlled by an internal 22A to change a capacitor slowly. It clamps the ramping of reference voltage at the input of error amplifier and the pulse-width of the output driver slowly. Under Voltage and Over Voltage Protection The voltage at FB pin is monitored and protected against OC (over current), and OV (over voltage). The UV threshold is 0.5V and OV-threshold is 1.0V. Both UV/OV detection have 30ms triggered delay. When OC or UV trigged, a hiccup re-start sequence will be initialized, as shown in Figure 3. Only 3 times of trigger are allowed to latch off. Hiccup is disabled during soft-start interval. Shutdown Pulling low the SS pin by a small single transistor can shutdown the RT9209/A PWM controller as shown in typical application circuit.
C2 1uF
VCC BOOT C1 UGATE 0.1uF + -
D1 +
5V
A VCC LGATE
RT9209/A
Figure 1. Single 5V power Supply Operation Dual Power Operation The RT9209/A is designed to supply a regulated 6.5V at VCC pin automatically when BOOT pin is powered by a 12V. In a system with ATX 5V/12V power supply, the RT9209/A is ideal for higher current applications due to the higher gate driving capability, VUGATE = 12V and VLGATE = 6.5V. A RC (10/1F) filter is also recommended at BOOT pin to prevent the ringing induced from fast poweron, as shown in Figure 2.
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RT9209/A
COUNT = 1
Internal
COUNT = 2
COUNT = 3
4V
SS
Q
2V 0V OVERLOAD
L
INDUCTOR CURRENT
VI
D
C
R
VO
APPLIED
C.C.M.
0A T0 T1 T2 TIME T3
TS
TON
TOFF VI - VO
Figure 3 Inductor Selection The RT9209/A was designed for VIN = 5V, step-down application mainly. Figure 4 shows the typical topology and waveforms of step-down converter. The ripple currents of inductor can be calculated as follows:
ILRIPPLE = (5V - VOUT) L x TON
iQ VL
- VO iL uQ uIL
IL = IO
Because operation frequency is fixed at 260kHz/400kHz,
TON = 3.85 x VOUT 5V , or 2.5 x VOUT 5V
IQ
The VOUT ripple is
VOUT
RIPPLE = ILRIPPLE x ESR
iD ID
ESR is output capacitor equivalent series resistor
Figure 4
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RT9209/A
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 RT9209/A 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 = 35 dB x LC filter pole PO = 1 2 1 2 5 1.6 x x LC x 0.8 VOUT Feedback Divider The reference of RT9209/A is 0.8V. The output voltage can be set using a resistor based divider as shown in Figure 8. Put the R1 and R2 as close as possible to FB pin and R2 should less than 1 k to avoid noise coupling.
VIN
Reference Voltage Because RT9209/A use a low 35dB gain error amplifier, shown in Figure 6. 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 7.
R2 FB + R1 1K REF 0.8V 56K EA + RAMP 1.75V
+ PWM -
Figure 6
0.82 VIN = 5V 0.81
FB (V)
0.80 0.79 0.78 0.5
1
1.5
2
2.5 3 VOUT (V)
3.5
4
4.5
Figure 7
Error Amp pole PA = 300kHz ESR zero ZO = x x ESR x C
The RT9209/A Bode plot as shown Figure 5 is stable in most of application conditions.
VOUT = 3.3V COUT = 1500uF(33m) L = 2uH VOUT = 1.5V PO = 2.9kHz VOUT = 2.5V ZO = 3.2kHz VOUT = 3.3V
A
L VOUT
+
40 30 20
COUT
R1 RT9209/A FB
Loop Gain 10
R2
100
1k
10k
100k
1M
Figure 5
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Figure 8
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RT9209/A
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 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 RT9209/A. 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 9 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 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 5V Q1 IQ2 Q2
+
IL A
+
VOUT
+
CIN GND
COUT
LOAD
GND LGATE VCC RT9209/A UGATE FB
Figure 9
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RT9209/A
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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