Techcode® ÓÉ Foxit PDF Editor ±à¼°æȨËùÓÐ (c) by Foxit Software Company, 2004 ½öÓÃÓÚÆÀ¹À¡£ DATASHEET Single Buck Voltage Mode PWM Controller TD1720 汪工 ＴＥＬ：１３８２８７１９４１０ ＱＱ：１９２９７９４２３８ General Description Features The TD1720 is a voltage mode, fixed 300kHz switching frequency, synchronous buck converter. The TD1720 allows wide input voltage that is either a single 5~12V or two supply voltage(s) for various applications. A power‐on‐reset (POR) circuit monitors the VCC supply voltage to prevent wrong logic controls. A builtin soft‐start circuit prevents the output voltages from overshoot as well as limits the input current. An internal 0.8V temperature‐compensated reference voltage with high accuracy is designed to meet the requirement of low output voltage applications. The TD1720 provides excellent output voltage regulations against load current variation. The controller’s over‐current protection monitors the output current by using the voltage drop across the RDS(ON) of low‐side MOSFET, eliminating the need for a current sensing resistor that features high efficiency and low cost. In addition, the TD1720 also integrates excellent protection functions: The over‐voltage protection (OVP) , under‐ voltage protection (UVP). OVP circuit which monitors the FB voltage to prevent the PWM output from overvoltage,and UVP circuit which monitors the FB voltage to prevent the PWM output from under‐voltage or short‐circuit.The TD1720 is available in SOP‐8P and TDFN3x3‐10 packages. z z z z z z z z z z z z z z z Applications z z z z z z Wide 5V to 12V Supply Voltage Power‐On‐Reset Monitoring on VCC Excellent Output Voltage Regulations 0.8V Internal Reference ±1% Over‐Temperature Range Integrated Soft‐Start Voltage Mode PWM Operation with External Compensation Up to 90% Duty Ratio for Fast Transient Response Constant Switching Frequency 300kHz ±10% Drive Dual Low Cost N‐MOSFETs with Adaptive Dead‐Time Control 50% Under‐Voltage Protection 125% Over‐Voltage Protection Adjustable Over‐Current Protection Threshold Using the RDS(ON) of Low‐Side MOSFET Shutdown Control by COMP Power Good Monitoring (TDFN‐10 3mmx3mm Package Only) SOP‐8P and TDFN3x3‐10 Packages Lead Free and Green Devices Available (RoHS Compliant) Graphic Cards DSL, Switch HUB Wireless Lan Notebook Computer Mother Board LCD Monitor/TV December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 1 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Pin Assignments PIN NO. SOP‐8P TDFN3x3‐10 NAME 1 1 BOOT 2 3 2 4 UGATE GND 4 5 LGATE 5 6 7 6 7 8 FUNCTION This pin provides the bootstrap voltage to the high‐side gate driver for driving the N‐channel MOSFET. An external capacitor from PHASE to BOOT, an internal diode, generates the bootstrap voltage for the high‐side d ( ) High‐side Gate Driver Output. This pin is the gate driver for high‐side Signal and Power ground. Connecting this pin to system ground. Low‐side Gate Driver Output and Over‐Current Setting Input. This pin is the gate driver for low‐side MOSFET. It also used to set the maximum inductor current. Refer to the section in “Function Description” for detail. VCC Power Supply Input. Connect a nominal 5V to 12V power supply voltage to this pin. A power‐on‐reset function monitors the input voltage at this pin. It is recommended that a decoupling capacitor (1 to 10 F) is connected to GND for noise decoupling. FB Feedback Input of Converter. The converter senses feedback voltage via FB and regulates the FB voltage at 0.8V. Connecting FB with a resistor‐divider from the output sets the output voltage of the converter. COMP This is a multiplexed pin. During soft‐start and normal converter operation, this pin represents the output of the error amplifier. It is used to compensate the regulation control loop in combination with the FB pin. Pulling COMP low (VDISABLE = 0.4V max.) will shut down the controller. When the pull‐down device is released, the COMP pin will start to rise. When the COMP pin rises above the VDISABLE trip point, the TD1720 will begin a new initialization and soft‐start cycle. December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 2 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 8 3 PHASE This pin is the return path for the high‐side gate driver. Connecting this pin to the high‐side MOSFET source and connect a capacitor to BOOT for the bootstrap voltage. This pin is also used to monitor the voltage drop across the low‐side MOSFET for over‐current protection. 9(Exposed Pad) 11(Exposed Pad) GND Thermal Pad. Connect this pad to the system ground plan for good thermal conductivity. ‐ 9 POK POK is an open drain output used to indicate the status of the output voltage. Connect the POK pin to 5 to 12V through a pull‐high resistor. ‐ 10 NC No Connect Ordering Information TD1720 □ □ Circuit Type Packing: Blank：Tube R:Type and Reel Package M:SOP8-PP Q:TDFN December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 3 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Functional Block Diagram Functional Block Diagram of TD1720 December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 4 Techcode® DATASHEET TD1720 Single Buck Voltage Mode PWM Controller Absolute Maximum Ratings Parameter Symbol VVCC VBOOT VUGATE VLGATE VPHASE TJ TSTG Rating VCC Supply Voltage (VCC to GND) BOOT Supply Voltage (BOOT to PHASE) BOOT Supply Voltage (BOOT to GND) UGATE Voltage (UGATE to PHASE) LGATE Voltage (LGATE to GND) PHASE Voltage (PHASE to GND) FB and COMP to GND POK to GND Maximum Junction Temperature Storage Temperature Maximum Lead Soldering Temperature, 10 Seconds > 20ns < 20ns > 20ns < 20ns > 20ns < 20ns Unit ‐0.3 ~ 16 ‐0.3 ~ 16 ‐0.3 ~ 30 ‐0.3 ~ VBOOT+0.3 ‐5 ~ VBOOT+5 ‐0.3 ~ VVCC+0.3 ‐5 ~ VVCC+5 ‐0.3 ~ 16 ‐5 ~ 21 ‐0.3 ~ 7 ‐0.3~VCC+0.3 150 ‐65 ~ 150 260 TSDR Note: Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. These are V V V V V V V V V V V °C °C °C stress ratings only and functional operation of the device at these or any other conditions beyond those indicated under "recom- mended operating conditions" is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Recommended Operating Conditions Symbol VIN VVCC VOUT IOUT TA TJ Parameter VIN Supply Voltage VCC Supply Voltage Converter Output Voltage Converter Output Current Ambient Temperature Junction Temperature Range 3.3 ~ 13.2 4.5 ~ 13.2 0.8 ~ 5.5 0 ~ 20 ‐40 ~ 85 ‐40 ~ 125 Unit V V V A °C °C Note : Refer to the application circuit for further information. December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 5 Techcode® DATASHEET TD1720 Single Buck Voltage Mode PWM Controller Thermal Characteristics Symbol Parameter Thermal Resistance -Junction to Ambient Typical Value Unit 60 °C/W (Note 2) JA SOP-8P JA the component mounted on a high effective thermal conductivity test board in free air. Note :is measured with Electrical Characteristics Refer to the typical application circuit. These specifications apply over VVCC = 12V, TA = -40°C to 85°C, unless otherwise noted. Typical values are at TA = 25°C. Symbol Parameter Test Conditions INPUT SUPPLY VOLTAGE AND CURRENT IVCC VCC Supply Current (Shutdown UGATE and Mode) COMP=GND LGATE TD1720 Min. Typ. open; - VCC Supply Current UGATE and LGATE open POWER-ON-RESET(POR) Rising VCC POR Threshold 3.8 VCC POR Hysteresis 0.3 OSCILLATOR FOSC Oscillator Frequency 270 VOSC (Note 4) (1.2V~2.7V typical) DMAX Maximum Duty Cycle REFERENCE VREF Reference Voltage TA = -40 ~ 85°C 0.792 Converter Line/Load Regulation VCC=4.5~13.2V, IOUT = 0 ~ -0.2 ERROR AMPLIFIER - gm Transconductance (Note ) RL = 10k , CL = 10pF - Open-Loop Bandwidth (Note ) FB Input Leakage Current COMP High Voltage COMP Low Voltage Maximum COMP Source Current VFB = 0.8V RL = OPEN RL = OPEN VCOMP = 2V Maximum COMP Sink Current VCOMP = 2V - - Max. Unit - 700 uA 2 3 mA 4.1 0.5 4.4 0.6 V V 300 1.5 - 330 - 90 kHz V % 0.8 - 0.808 0.2 V % 667 - A/V 20 - MHz - 3 1.5 200 0.1 - - - uA 200 - V uA December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 6 Techcode® DATASHEET TD1720 Single Buck Voltage Mode PWM Controller Electrical Characteristics(Cont.) Refer to the typical application circuit. These specifications apply over VVCC = 12V, TA = -40°C to 85°C, unless otherwise noted. Typical values are at TA = 25°C. Symbol Parameter Test Conditions GATE DRIVERS High‐Side Gate Driver SourceVBOOT‐GND= 12V, VUGATE‐PHASE = 6V High‐Side Gate Driver Sink Current VBOOT‐GND= 12VUGATE‐PHASE = 6V Low‐Side Gate Driver SourceVVCC = 12V, VLGATE‐GND = 6V Low‐Side Gate Driver Sink Current VVCC = 12V, VLGATE‐GND = 6V TD Dead‐Time (Note 4) PROTECTIONS VFB_UV FB Under‐Voltage Protection TripPercentage of VREF Under‐Voltage Debounce Interval Under‐Voltage Protection Enable The same as soft ‐start interval Delay VFB_OV FB Over‐Voltage Protection TripVFB rising FB Over‐Voltage Protection Over‐Voltage Debounce Interval VOCP_MAX Built‐in Maximum OCP Voltage IOCSET OCSET Current Source SOFT‐START VDISABLE Shutdown Threshold of VCOMP TSS Internal Soft‐Start Interval (Note 4) POWER OK INDICATOR (POK) (ONLY FOR TDFN3X3‐10 PACKAGE) IPOK POK Leakage Current VPOK=5V VFB is from low to target value (POK Goes High) VPOK POK Threshold VFB Falling, POK Goes Low VFB Rising, POK Goes Low POK Delay Time TD1720 Min. Typ. Max. ‐ ‐ ‐ ‐ ‐ 1.0 1.1 1.5 1.8 30 ‐ ‐ ‐ ‐ ‐ 40 ‐ 45 2 50 ‐ % s 1 1.5 2 ms 115 ‐ ‐ 350 9 125 5 2 ‐ 10 135 ‐ ‐ ‐ 11 % % s mV uA ‐ 1 ‐ 1.5 0.4 2 V ms ‐ 0.1 1 uA 85 90 95 % 45 120 1 50 125 3 55 130 5 % % ms Unit A A ns Note 4: Guaranteed by design, not production tested. December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 7 Techcode® Single Buck Voltage Mode PWM Controller DATASHEET TD1720 Typical Operating Characteristics December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 8 Techcode® Single Buck Voltage Mode PWM Controller DATASHEET TD1720 Typical Operating Characteristics(Cont.) December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 9 Techcode® Single Buck Voltage Mode PWM Controller DATASHEET TD1720 Typical Operating Characteristics(Cont.) December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 10 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Typical Application Circuit TD1720 12V Application Circuit TD1720 5V Application Circuit December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 11 Techcode® DATASHEET TD1720 Single Buck Voltage Mode PWM Controller Function Description Power‐On‐Reset (POR) A resistor (ROCSET), connected from the LGATE/OCSET to GND, The Power-On-Reset (POR) function of TD1720 continually programs the over-current trip level. Before the IC initiates a monitors the input supply voltage (VCC) and ensures that the IC soft-start process, an internal current source, IOCSET (10μA typical), has sufficient supply voltage and can work well. The POR function flowing through the ROCSET develops a voltage (VROCSET) across the initiates a soft-start process while the VCC voltage just exceeds ROCSET. The device holds VROCSET and stops the current source IOCSET the POR threshold; the POR function also inhibits the operations during normal operation. When the voltage across the low-side of the IC while the VCC voltage falls below the POR threshold. MOSFET exceeds the VROCSET, the TD1720 turns off the highside Soft‐Start and low-side MOSFET,and the device will enters hiccup mode The TD1720 builds in a soft-start function about 1.5ms (Typ.) until the over-current phenomenon is released. interval, which controls the output voltage rising as well as limiting The TD1720 has an internal OCP voltage, VOCP_MAX, and the value the current surge at the start-up. During soft-start, an internal ramp is 0.35V (minimum). When the ROCSET x IOCSET exceed 0.35V or the voltage connected to the one of the positive inputs of the error ROCSET is floating or not connected, the VROCSET will be the default amplifier replaces the reference voltage (0.8V typical) until the value 0.35V. The over current threshold would be 0.35V across ramp voltage reaches the reference voltage. The soft-start circuit low-side MOSFET. The threshold of the valley inductor interval is shown as figure 1. The UVP function enable delay is current-limit is therefore given by: from t2 to t3. For the over-current is never occurred in the normal operating load range, the variation of all parameters in the above equation should be considered: - The RDS(ON) of low-side MOSFET is varied by temperature and gate to source voltage. Users should determine the maximum RDS(ON) by using the manufacturer’s datasheet. Over‐Current Protection of the PWM Converter - The minimum IOCSET (9μA) and minimum ROCSET should be used in the above equation. The over-current function protects the switching converter against - Note that the ILIMIT is the current flow through the lowside over-current or short-circuit conditions. The controller senses the MOSFET; ILIMIT must be greater than valley inductor current which inductor current by detecting the drainto-source voltage which is is output current minus the half of inductor ripple current. the product of the inductor’s current and the on-resistance of the low-side MOSFET during it’s on-state. This method enhances the converter’s efficiency and reduces cost by eliminating a current sensing resistor required. Where ΔI = output inductor ripple current - The overshoot and transient peak current also should be considered. December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 12 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Function Description(Cont.) Under‐Voltage Protection Adaptive Shoot‐Through Protection of the PWM Converter The under-voltage function monitors the voltage on FB (VFB) by The gate drivers incorporate an adaptive shoot-through protection Under-Voltage (UV) comparator to protect the PWM converter to prevent high-side and low-side MOSFETs from conducting against short-circuit conditions. When the VFB falls below the falling simultaneously and shorting the input supply. This is accomplished UVP threshold (50% VREF), a fault signal is internally generated by ensuring the falling gate has turned off one MOSFET before the and the device turns off highside and low-side MOSFETs. The other is allowed to rise. device will enters hiccup mode until the under-voltage During turn-off the low-side MOSFET, the LGATE voltage is phenomenon is released. monitored until it is below 1.5V threshold, at which time the Over‐Voltage Protection (OVP) of the PWM Converter UGATE is released to rise after a constant delay. During turn-off of The over-voltage protection monitors the FB voltage to prevent the the high-side MOSFET, the UGATE-to-PHASE voltage is also output from over-voltage condition. When the output voltage rises monitored until it is below 1.5V threshold, at which time the LGATE above 125% of the nominal output voltage, the TD1720 turns off is released to rise after a constant delay. the high-side MOSFET and turns on the low-side MOSFET until Power OK Indicator the output voltage falls below the falling OVP threshold. The TD1720 features an open-drain POK output pin to indicate Shutdown and Enable one of the IC's working statuses including soft-start, under-voltage The TD1720 can be shut down or enabled by pulling low the fault, over-current fault. In normal operation, when the output voltage on COMP. The COMP is a dual-function pin. During voltage rises 90% of its target value, the POK goes high. When the normal operation, this pin represents the output of the error output voltage outruns 50% or 125% of the target voltage, POK amplifier. It is used to compensate the regulation control loop in signal will be pulled low immediately. combination with the FB pin. Pulling the COMP low (VDISABLE = 0.4V maximum) places the controller into shutdown mode which UGATE and LGATE are pulled to PHASE and GND respectively. When the pull-down device is released, the COMP voltage will start to rise. When the COMP voltage rises above the VDISABLE threshold, the TD1720 will begin a new initialization and soft-start process. December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 13 Techcode® DATASHEET TD1720 Single Buck Voltage Mode PWM Controller Application Information Output Voltage Selection where Fs is the switching frequency of the regulator. The output voltage can be programmed with a resistive divider. Use 1% or better resistors for the resistive divider is A tradeoff exists between the inductor’s ripple current and the recommended. The FB pin is the inverter input of the error regulator load transient response time. A smaller inductor will give amplifier, and the reference voltage is 0.8V. The output voltage is the regulator a faster load transient response at the expense of determined by: higher ripple current and vice versa. The maximum ripple current occurs at the maximum input voltage. A good starting point is to choose the ripple current to be approximately 30% of the Where R1 is the resistor connected from VOUT to FB and R2 is the maximum output current. resistor connected from FB to the GND. Once the inductance value has been chosen, selecting an inductor Output Capacitor Selection is capable of carrying the required peak current without going into The selection of COUT is determined by the required effective series saturation. In some types of inductors, especially core that is make resistance (ESR) and voltage rating rather than the actual of ferrite, the ripple current will increase abruptly when it saturates. capacitance requirement. Therefore, selecting high performance This will result in a larger output ripple voltage. low ESR capacitors is intended for switching regulator Compensation applications. In some applications.multiple capacitors have to be The output LC filter of a step down converter introduces a double paralleled to achieve the desired ESR value. If tantalum capacitors pole, which contributes with -40dB/decade gain slope and 180 are used, make sure they are surge tested by the manufactures. If degrees phase shift in the control loop. A compensation network in doubt,consult the capacitors manufacturer. between COMP pin and ground should be added. The simplest Input Capacitor Selection loop compensation network is shown in Figure 5.The output LC The input capacitor is chosen based on the voltage rating and the filter consists of the output inductor and output capacitors. The RMS current rating. For reliable operation,select the capacitor transfer function of the LC filter is given by: voltage rating to be at least 1.3 times higher than the maximum input voltage. The maximum RMS current rating requirement is approximately IOUT/2 where IOUT is the load current. During power up, the input capacitors have to handle large amount of surge The poles and zero of this transfer function are: current.If tantalum capacitors are used, make sure they are surge tested by the manufactures. If in doubt, consult the capacitors manufacturer.For high frequency decoupling, a ceramic capacitor between 0.1μF to 1μF can connect between VCC and ground pin. Inductor Selection The inductance of the inductor is determined by the output voltage The FLC is the double poles of the LC filter, and FESR is the zero requirement. The larger the inductance, the lower the inductor’s introduced by the ESR of the output capacitor. current ripple. This will translate into lower output ripple voltage. The ripple current and ripple voltage can be approximated by: December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 14 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Application Information(Cont.) The compensation circuit is shown in Figure 5. R2 and C2 introduce a zero and C1 introduces a pole to reduce the switching noise. The transfer function of error amplifier is given by: Figure 2. The Output LC Filter The pole and zero of the compensation network are: Figure 3. The LC Filter Gain & Frequency The PWM modulator is shown in Figure 4. The input is the output of the error amplifier and the output is the PHASE node. The transfer function of the PWM modulator is given by: Figure 5. Compensation Network The closed loop gain of the converter can be written as: Figure 6 shows the converter gain and the following guidelines will help to design the compensation network. 1.Select the desired zero crossover frequency FO: (1/5 ~ 1/10) x FSW >FO>FZ Use the following equation to calculate R2: Figure 4. The PWM Modulator Where:gm = 667μA/V December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 15 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Application Information(Cont.) 2. Place the zero FZ before the LC filter double poles FLC: where IOUT is the load current FZ = 0.75 x FLC TC is the temperature dependency of RDS(ON) Calculate the C2 by the equation: FSW is the switching frequency tsw is the switching interval D is the duty cycle 3. Set the pole at the half the switching frequency: Note that both MOSFETs have conduction losses while the upper FP = 0.5xFSW MOSFET includes an additional transition loss. The switching Calculate the C1 by the equation: internal, tsw, is the function of the reverse transfer capacitance CRSS. Figure 7 illustrates the switching waveform internal of the MOSFET. The (1+TC) term factors in the temperature dependency of the RDS(ON) and can be extracted from the “RDS(ON) vs Temperature” curve of the power MOSFET. Figure 6. Converter Gain & Frequency MOSFET Selection The selection of the N-channel power MOSFETs is determined by the RDS(ON), reverse transfer capacitance (CRSS), and maximum output current requirement.The losses in the MOSFETs have two components: conduction loss and transition loss. For the upper Figure 7. Switching Waveform Across MOSFET and lower MOSFET, the losses are approximately given by the Layout Consideration following equations: In any high switching frequency converter, a correct layout is PUPPER = IOUT2 (1+ TC)(RDS(ON))D + (0.5)(Iout)(VIN)(tsw)FSW important to ensure proper operation of the regulator. With power PLOWER = IOUT2 (1+ TC)(RDS(ON))(1-D) devices switching at 300kHz,the resulting current transient will cause voltage spike across the interconnecting impedance and parasitic circuit elements. As an example, consider the turn-off transition of the PWM MOSFET. Before turn-off, the MOSFET is carrying the full load current. During turn-off, current stops flowing in the MOSFET and is free-wheeling by the lower MOSFET and parasitic diode. Any parasitic inductance of the circuit generates a December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 16 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Application Information(Cont) large voltage spike during the switching interval. In general, using - The input capacitor should be near the drain of the upper short and wide printed circuit traces should minimize MOSFET; the output capacitor should be near the loads. The input interconnecting imped ances and the magnitude of voltage spike. capacitor GND should be close to the output capacitor GND and And signal and power grounds are to be kept separate till the lower MOSFET GND. combined using ground plane construction or single point - The drain of the MOSFETs (VIN and PHASE nodes) should be a grounding. Figure 8. illustrates the layout, with bold lines indicating large plane for heat sinking. high current paths; these traces must be short and wide. - The ROCSET resistance should be placed near the IC as close as Components along the bold lines should be placed lose together. possible. Below is a checklist for your layout: - Keep the switching nodes (UGATE, LGATE, and PHASE) away from sensitive small signal nodes since these nodes are fast moving signals. Therefore, keep traces to these nodes as short as possible. - The traces from the gate drivers to the MOSFETs (UG and LG) should be short and wide. - Place the source of the high-side MOSFET and the drain of the low-side MOSFET as close as possible. Minimizing the impedance with wide layout plane between the two pads reduces the voltage bounce of the node. Decoupling capacitor, compensation component, the resistor dividers, and boot capacitors should be close their pins. (For example, place the decoupling ceramic capacitor near the drain of Figure 8. Layout Guidelines the high-side MOSFET as close as possible. The bulk capacitors are also placed near the drain). December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 17 Techcode® Single Buck Voltage Mode PWM Controller DATASHEET TD1720 Package Information SOP8-PP Package Outline Dimensions December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 18 Techcode® DATASHEET TD1720 Single Buck Voltage Mode PWM Controller Package Information TDFN3x3-10 December, 20, 2011 Techcode Semiconductor Limited www.tongchuangwei.com 19 Techcode® DATASHEET Single Buck Voltage Mode PWM Controller TD1720 Design Notes December, 20, 2011 Techcode Semiconductor Limited www.techcodesemi.com 20