TL1454CPW Datasheet, PDF (6/25 Pages) Texas Instruments – DUAL-CHANNEL PULSE-WIDTH-MODULATION (PWM) CONTROL CIRCUIT

TL1454, TL1454Y
DUAL-CHANNEL PULSE-WIDTH-MODULATION (PWM)
CONTROL CIRCUIT
SLVS086B – APRIL 1995 – REVISED NOVEMBER 1997
dead-time control (DTC) and soft start
The two PWM channels have independent dead-time control inputs so that the maximum power-switch duty
cycles can be limited to less then 100%. The dead-time is set with a voltage applied to DTC; the voltage is
typically obtained from a resistive divider connected between the reference and ground as shown in Figure 4.
Soft start is implemented by adding a capacitor between REF and DTC.
+ * ǒ * Ǔ * The voltage, VDT, required to limit the duty cycle to a maximum value is given by:
VDT VO(max) D VO(max) VO(min) 0.65
where V_O(max) and V_O(min) are obtained from Figure 9, and D is the maximum duty cycle.
Predicting the regulator startup or rise time is complicated because it depends on many variables, including:
input voltage, output voltage, filter values, converter topology, and operating frequency. In general, the output
will be in regulation within two time constants of the soft-start circuit. A five-to-ten millisecond time constant
usually works well for low-power converters.
The DTC input can be grounded in applications where achieving a 100% duty cycle is desirable, such as a buck
converter with a very low input-to-output differential voltage. However, grounding DTC prevents the
implementation of soft start, and the output voltage overshoot at power-on is likely to be very large. A better
arrangement is to omit R_DT1 (see Figure 4) and choose R_DT2 = 47 kΩ. This configuration ensures that the duty
cycle can reach 100% and still allows the designer to implement soft start using C_SS.
16 _REF
CSS
RDT1
TL1454
DTC
RDT2
Figure 4. Dead-Time Control and Soft Start
PWM comparator
Each of the PWM comparators has dual inverting inputs. One inverting input is connected to the output of the
error amplifier; the other inverting input is connected to the DTC terminal. Under normal operating conditions,
when either the error-amplifier output or the dead-time control voltage is higher than that for the PWM triangle
wave, the output stage is set inactive (OUT1 low and OUT2 high), turning the external power stage off.
undervoltage-lockout (UVLO) protection
The undervoltage-lockout circuit turns the output circuit off and resets the SCP latch whenever the supply
voltage drops too low (to approximately 2.9 V) for proper operation. A hysteresis voltage of 200 mV eliminates
false triggering on noise and chattering.
short-circuit protection (SCP)
The TL1454 SCP function prevents damage to the power switches when the converter output is shorted to
ground. In normal operation, SCP comparator 1 clamps SCP to approximately 185 mV. When one of the
converter outputs is shorted, the error amplifier output (COMP) will be driven below 1 V to maximize duty cycle
and force the converter output back up. When the error amplifier output drops below 1 V, SCP comparator 1
releases SCP, and capacitor, C_SCP, which is connected between SCP and GND, begins charging. If the
error-amplifier output rises above 1 V before C_SCP is charged to 1 V, SCP comparator 1 discharges C_SCP and
normal operation resumes. If C_SCP reaches 1 V, SCP comparator 2 turns on and sets the SCP latch, which turns
off the output drives and resets the soft-start circuit. The latch remains set until the supply voltage is lowered
to 2 V or less, or C_SCP is discharged externally.
6
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
• POST OFFICE BOX 1443 HOUSTON, TEXAS 77251–1443