PWM Controller With SCP / DTC Function

Transcription

1 PWM Controller With / DTC Function FP51 General Description The FP51 is a single chip pulse-width-modulation controller composed of an open collector transistor output, an error amplifier and duty control comparators (DTC). The FP51 contains a 1.V precision voltage reference regulator, under-voltage lockout circuit (UVLO), short circuit protection circuit (). It is applied to offer space and low cost in many applications such as the DC / DC converter and backlight inverter. Using FP51, it is easy to complete a power conversion regulator design with few external components. The circuit diagram of the typical application example is shown as below. Features Wide Operating Voltage Range: 3.6~4V Reference Voltage Precision: 2% Oscillator Frequency: 5KHz Max. Output sink current up to 1mA Low Quiescent Supply Current Variable Dead-time Control (DTC) UVLO Protection Function Protection Function Package: SOP-8L Applications Graphic Card Telecom and Datacom Applications High Power DC-DC Regulators Typical Application Circuit VIN VOUT VCC RT DTC VO FP51A FB GND COMP 1/14

2 Function Block Diagram VCC RT DTC OUT UVLO 1V Reference Voltage 1V 2.5V Idt 1.5V OSC Error AMP COMP 1 PWM / DTC COMP FB COMP COMP 2 GND Pin Descriptions SOP-8L Name No. I / O Description OUT 1 O Open Collector Transistor Output V CC 2 P IC Power Supply FP51 9Fa-86L COMP 3 O Error Amplifier Output FB 4 I Error Amplifier Inverting Input 5 I Short Circuit Protection Input DTC 6 I Duty Control Input RT 7 I Oscillator Frequency Control Input GND 8 P IC Ground 2/14

3 Marking Information SOP-8L FP51 - Halogen Free Lot Number Internal ID Per-Half Month Year Halogen Free: Halogen free product indicator Lot Number: Wafer lot number s last two digits For Example: TB 86 Internal ID: Internal Identification Code n Per-Half Month: Production period indicated in half month time unit For Example: January A (Front Half Month), B (Last Half Month) Year: Production year s last digit February C (Front Half Month), D(Last Half Month) 3/14

4 Ordering Information Part Number Operating Temperature Package MOQ Description FP51DR-LF -2 C ~ +85 C SOP-8L 25EA Tape & Reel Absolute Maximum Ratings Parameter Symbol Conditions Min. Typ. Max. Unit Power Supply Voltage V CC 4 V Differential Input Voltage V ID 2 V Collector Output Voltage Vo 4 V Collector Output Current Io 15 ma Maximum Junction Temperature T J 15 Junction to Ambient Thermal Resistance Power Dissipation (SOP-8L SOP-8L Package +175 / W T A =25 65 mw package) T A =7 55 mw Storage Temperature Range C Lead Temperature (Soldering, 1 sec) +26 C IR Re-flow Soldering Curve 4/14

5 Recommended Operating Conditions Parameter Symbol Conditions Min. Typ. Max. Unit Supply Voltage V CC V Operating Temperature C Operating Junction Temperature C DC Electrical Characteristics (V CC = 6V, T A =25 C, f OSC =7KHz unless otherwise specified) Parameter Symbol Conditions Min. Typ. Max. Unit Reference Output Voltage V REF COMP Connected to FB V Input Regulation V REF V CC =3.6 V to 4 V mv Output Voltage Change with V REF / T A =-2 to mv / V Temperature V REF T A = 25 to mv / V Under Voltage Lockout Upper Threshold Voltage V upper 3 V Lower Threshold Voltage V low 2.8 V Hysteresis V hys 1 2 mv Reset Threshold Voltage V reset V Short-Circuit Protection Threshold Voltage V TH V Voltage, Latched V LATCH No Pullup 2.4 V Voltage, UVLO Operation V OPR mv Voltage, UVLO Standby V STANDBY No Pullup 6 12 mv Input Source Current I SOURCR T A = μa Comparator 1 Threshold Voltage V COMP (TH) 1.5 V Oscillator Frequency f R T =1K 7 KHz Standard Deviation of Frequency Frequency Change with Voltage f 15 KHz f / V V CC =3.6V to 4V 1 KHz Frequency Change with Temperature f / T T A =-2 to KHz T A = 25 to KHz Voltage at RT V RT 1 V 5/14

6 DC Electrical Characteristics (Cont.) Parameter Symbol Conditions Min. Typ. Max. Unit Duty Control Output (Source) Current I SOURCE V (DT) =1.5V.9 I RT 1.2 I RT μa Input Threshold Voltage Error Amplifier V TH Duty Cycle=%.5.7 V Duty Cycle=1% V Input Voltage V IN V CC =3.6V to 4V 1.5 V Input Bias Current V BIAS na Output Voltage Swing Positive V POS V Output Voltage Swing Negative V NEG.3.4 V Open-loop Voltage Amplification A VO 8 db Unity-Gain Bandwidth BW U 1.5 MHz Output(Sink)Current I SINK V I (FB) =1.2V, COMP=1V 6 11 μa Output(Source)Current I SOURCE V I (FB) =.8V, COMP=1V μa Output Output Saturation Voltage V SAT I O =1mA V Off-state Current I OFF V O =4V, V CC = 1 μa V O =4V 1 μa Short-circuit Output Current I SC V O =6V 4 ma Total Device Standby Supply Current Off State I STANDBY ma Average Supply Current I AVE R T =1k ma 6/14

7 Function Description Voltage Reference A 2.5-V regulator operating from V CC is used to power the internal circuitry of the FP51. A resistive divider provides 1V reference for the error amplifier and the circuit. Error Amplifier The error amplifier compares a sample of the DC-DC converter s output voltage to the 1V reference and generates an error signal for the PWM comparator. The DC-DC converter s output voltage can be set by following expression: Vo = (1 + R1 / R2) (1 V) Compensatio n Network 3 COMP FP51A VI(FB) R1 R2 4 FB To PWM Comparator 8 GND Vref = 1 V Figure 1. Error-Amplifier Gain Setting The error amplifier s output is brought out as COMP for use in compensating the DC-DC converter s control loop for stability. Because the amplifier can only source 45 μa, the total DC load resistance should be 1 kω or larger. Oscillator / PWM The oscillator frequency (fosc) can be set from 2 khz to 5 khz by connecting a resistor between RT and GND. Acceptable resistor values range from 15 kω to 25 kω. The oscillator frequency can be determined by using the graph shown in Figure 5. The oscillator output is a triangular wave with a minimum value of approximately.7 V and a maximum value of approximately 1.3 V. The PWM comparator compares the error amplifier s output voltage and the DTC input voltage to the triangular wave. It turns the output transistor off whenever the triangular wave is greater than the lesser of the two inputs. 7/14

8 Duty Control (DTC) DTC provides a means of limiting the output-switch duty cycle to a value less than 1%, which is critical for boost and flyback converters. A current source generates a reference current (I DT ) at DTC that is nominally equal to the current at the oscillator timing terminal, RT. Connecting a resistor between DTC and GND generates a maximum duty reference voltage (V DT ), which the PWM/DTC comparator compares to the oscillator triangle wave as described in the previous section. Nominally, the maximum duty cycle is % when V DT is smaller than.7 V and 1% when V DT is 1.3 V or greater. Because the triangle wave amplitude is a function of frequency and the source impedance of RT is relatively high (125 Ω), choosing R DT for a specific maximum duty cycle, D, is accomplished using the following equation. The voltage limits for the frequency in question can be found in Figure 11 (V osc max and V osc min are the maximum and minimum oscillator levels): R DT = ( R T ) [ D ( V osc max - V osc min) + V osc min ] Where R DT and R T are in ohms, D in decimal Soft start can be implemented by paralleling the DTC resistor with a capacitor (C DT ) as shown in Figure 2. During soft start, the voltage at DTC is derived by the following equation: V DT I DT R DT 1 - e - t / R DT C DT 6 DTC FP51A C DT R DT Figure 2. Soft-Start Circuit lf the DC-DC converter must be in regulation within a specified period of time, the time constant, R DT C DT, should be t /3 to t /5. The FP51 remains off when V DT <.7 V, the minimum OSC ramp value. C DT is discharged every time UVLO or becomes active. 8/14

9 Under Voltage Lockout (UVLO) Protection The under voltage lockout circuit turns the output transistor off and resets the latch whenever the supply voltage drops too low (approximately 3 V at 25 C) for proper operation. A hysteresis voltage of 2 mv eliminates false triggering on noise and chattering. Short-circuit Protection () The FP51 includes short-circuit protection (see Figure 3), which turns the power switch off to prevent damage when the converter output is shorted. When activated, the prevents the switch from being turned on until the internal latching circuit is reset. The circuit is reset by reducing the input voltage until UVLO becomes active or until the terminal is pulled to ground externally. When a short circuit occurs, the error-amplifier output at COMP rises to increase the power-switch duty cycle in an attempt to maintain the output voltage. comparator 1 starts an RC timing circuit when COMP exceeds 1.5V. If the short is removed and the error-amplifier output drops below 1.5V before time out, normal converter operation continues. If the fault is still present at the end of the time-out period, the timer sets the latching circuit and turns off the FP51 output transistor. 2.5 V Rscp 185 kω Cscp 5 From Error Amp 1.5 V Comparator 1 12 kω Q1 Comparator 2 Vref = 1 V To Output Drive Logic Q2 Figure 3. Circuit The timer operates by charging an external capacitor (C), connected between the terminal and ground, towards 2.5V through a 185-kΩ resistor (R). The circuit begins charging from an initial voltage of approximately 185 mv and times out when the capacitor voltage reaches 1V. The output of comparator 2 then goes high, turns on Q2, and latches the timer circuit. The expression for setting the time period is derived from the following equation: V = ( )(1 e -t/τ ) Where τ = R C 9/14

10 The end of the time-out period, t, occurs when V = 1 V. Solving for C yields: C = t Where t is in seconds, C in μf. t must be much longer (generally 1 to 15 times) than the converter start-up period or the converter will not start. Output Transistor The output of the FP51 is an open-collector transistor with a maximum collector current rating of 1mA and a voltage rating of 4V. The output is turned on under the following conditions: the oscillator triangle wave is lower than both the DTC voltage and the error-amplifier output voltage, the UVLO circuit is inactive, and the short-circuit protection circuit is inactive. PARAMETER MEASUREMENT INFORMATION DTC OSC COMP 1.5 V PWM / DTC Comparator OUT Comparator 1 Comparator 2 Timing period 1. V V Vcc 3. V Figure 4. PWM Timing Diagram 1/14

11 Typical Operating Characteristics 1M OSCILLATOR FREQUENCY TIMING RESISTANCE Vcc = 6 V TA =25 o C 75 7 OSCILLATOR FREQUENCY AMBIENT TEMPERATURE Rt =1kΩ fosc - Oscillator Frequency - Hz 1 K fosc - Oscillator Frequency - khz K 1 K 1 K 1 M Rt - Timing Resistance - FG[ TA- Ambient Temperature - Figure 5 Figure 6 Vref - Reference Output Voltage - V REFERENCE OUTPUT VOLTAGE POWER-SUPPLY VOLTAGE TA =25 o C 1.4 FB and COMP Connected Together 1.2! FGgVref - Reference Output Voltage Fluctuation - % REFERENCE OUTPUT VOLTAGE FLUCTUATION AMBIENT TEMPERATURE FB and COMP Connected Together Vcc - Power Supply Voltage - V TA - Ambient Temperature - Figure 7 Figure 8 2. AVERAGE SUPPLY CURRENT POWER-SUPPLY VOLTAGE 1.3 AVERAGE SUPPLY CURRENT AMBIENT TEMPERATURE T = 25 o C R A t =1kΩ 1.2 Rt =1kΩ Icc - Average Supply Current - ma Icc - Average Supply Current - ma Vcc - Power-Supply Voltage - V TA- Ambient Temperature - Figure 9 Figure 1 11/14

12 PWM Triangle Wave Amplitude Voltage - V PWM TRIANGLE WAVE AMPLITUDE VOLTAGE OSCILLATOR FREQUENCY Vcc = 6 V TA = 25 o C Vosc max (1% duty cycle) Vosc min (zero duty cycle) Vo - Error Amplifier Output Voltage - V ERROR AMPLIFIER OUTPUT VOLTAGE OUTPUT (SINK) CURRENT T = 25 o C V A I(FB) = 1.2 V Vcc = 6 V 1 K 1 K 1M 1 M fosc - Oscillator Frequency - Hz Io - Output (Sink) Current - ma Figure 11 Figure ERROR AMPLIFIER OUTPUT VOLTAGE vs OUTPUT (SOURCE) CURRENT 2.4 ERROR AMPLIFIER OUTPUT VOLTAGE AMBIENT TEMPERATURE Vo - Error Amplifier Output Voltage - V T = 25 o C V A I(FB) =.8 V Vcc = 6 V Vo - Error Amplifier Output Voltage - V VI(FB) =.8 V No Load Io - Output (Source) Current - μa TA - Ambient Temperature - Figure 13 Figure ERROR AMPLIFIER OUTPUT VOLTAGE AMBIENT TEMPERATURE 12 OUTPUT DUTY CYCLE DTC VOLTAGE Vo - Error Amplifier Output Voltage - mv VI(FB) = 1.2 V No Load Output Duty Cycle - % TA = 25 o C Rt =1kΩ TA - Ambient Temperature DTC Voltage - V Figure 15 Figure 16 12/14

13 12 TIME-OUT PERIOD CAPACITANCE - 6 DTC OUTPUT CURRENT RT OUTPUT CURRENT t(scp) - Time-Out Period - ms TA = 25 o C Rt =1kΩ Io(DT) - DTC Output Current - μa DTC Voltage = 1.3V TA = 25 o C Cscp - Capacitance - nf Io - RT Output Current - μa Figure 17 Figure 18 2 OUTPUT SATURATION VOLTAGE OUTPUT (SINK) CURRENT TA = 25 o C Vce - Output Saturation Voltage - V Io - Output (Sink) Current - ma Figure 19 13/14

14 Package Outline SOP-8L UNIT: mm Symbols Min. (mm) Max. (mm) A A A D E H L θ 8 Note: 1. Package dimensions are in compliance with JEDEC outline: MS-12 AA. 2. Dimension D does not include molding flash, protrusions or gate burrs. 3. Dimension E does not include inter-lead flash or protrusions. 14/14