TL5001_15 Datasheet, PDF(7/13 Page) Unisonic Technologies – PULSE-WIDTH-MODULATION CONTROL CIRCUIT

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TL5001_15 Datasheet, PDF (7/13 Pages) Unisonic Technologies – PULSE-WIDTH-MODULATION CONTROL CIRCUIT

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TL5001

ï® DETAILED DESCRIPTION(Cont.)

LINEAR INTEGRATED CIRCUIT

DEAD TIME CONTORL (DTC)

DTC provides a means of limiting the output-switch duty cycle to a value less than 100%, which is critical for boost

and flyback converters. A current source generates a reference current (IDT) at DTC that is nominally equal to the

current at the oscillator timing terminal, RT. Connecting a resistor between DTC and GND generates a dead-time

reference voltage (VDT), which the PWM/DTC comparator compares to the oscillator triangle wave as described in

the previous section. Nominally, the maximum duty cycle is 0% when VDT is 0.7V or less and 100% when VDT is

1.3V or greater. Because the triangle wave amplitude is a function of frequency and the source impedance of RT is

relatively high(1250â¦),choosing RDT for a specific maximum duty cycle, D, is accomplished using the following

equation and the voltage limits for the frequency in question as found in Fig. 11(VOSC(MAX) and VOSC(MIN) are the

maximum and minimum oscillator levels):

RDT=(RT +1250)[D(VOSC(MAX)-VOSC(MIN))+VOSC(MIN)]

Where

RDT and RT are in ohms, D in decimal

Soft start can be implemented by paralleling the DTC resistor with a capacitor (CDT) as shown in Fig. 2. During soft

start, the voltage at DTC is derived by the following equation:

VDTâIDTRDT(1-e(-t/RDTCDT))

6 DTC UTC

CDT

RDT TL5001

Fig. 2 Soft- Start Circuit

If the dc-to-dc converter must be in regulation within a specified period of time, the time constant, RDTCDT,

should be t0/3 to t0/5.The UTC TL5001 remains off unit VDTâ0.7V, the minimum ramp value. CDT is discharged

every time UVLO or SCP becomes active.

UNDERVOLTAGE-LOCKOUT (UVLO) PROTECTION

The undervoltage-lockout circuit turns the output transistor off and resets the SCP latch whenever the supply

voltage drops too low (approximately 3V at 25â) for proper operation. A hysteresis voltage of 200mV eliminates

false triggering on noise and chattering.

UNISONIC TECHNOLOGIES CO., LTD

www.unisonic.com.tw

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QW-R103-016.E

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