TL5001 Datasheet, PDF (3/23 Pages) Texas Instruments – PULSE-WIDTH-MODULATION CONTROL CIRCUITS

TL5001, TL5001A
PULSE-WIDTH-MODULATION CONTROL CIRCUITS
SLVS084E – APRIL 1994 – REVISED OCTOBER 1999
detailed description
voltage reference
A 2.5-V regulator operating from V_CC is used to power the internal circuitry of the TL5001 and TL5001A and
as a reference for the error amplifier and SCP circuits. A resistive divider provides a 1-V reference for the error
amplifier noninverting input which typically is within 2% of nominal over the operating temperature range.
error amplifier
The error amplifier compares a sample of the dc-to-dc converter output voltage to the 1-V reference and
generates an error signal for the PWM comparator. The dc-to-dc converter output voltage is set by selecting
the error-amplifier gain (see Figure 1), using the following expression:
V_O = (1 + R1/R2) (1 V)
Compensation
Network
R1
VI(FB)
R2
3 COMP
4 FB
TL5001/A
–
To PWM
Comparator
+
8 _GND
Vref = 1 V
Figure 1. Error-Amplifier Gain Setting
The error-amplifier output is brought out as COMP for use in compensating the dc-to-dc converter control loop
for stability. Because the amplifier can only source 45 µA, the total dc load resistance should be 100 kΩ or more.
oscillator/PWM
The oscillator frequency (f_osc) can be set between 20 kHz and 500 kHz by connecting a resistor between RT
and GND. Acceptable resistor values range from 15 kΩ to 250 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 0.7 V and a maximum value
of approximately 1.3 V. The PWM comparator compares the error-amplifier output voltage and the DTC input
voltage to the triangular wave and turns the output transistor off whenever the triangular wave is greater than
the lesser of the two inputs.
dead-time control (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 (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 dead-time 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 0 % when V_DT is 0.7 V or less and
100 % 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 (1250 Ω), choosing R_DT 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
Figure 11 (V_oscmax and V_oscmin are the maximum and minimum oscillator levels):
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