Datasheet
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Shutdown Options (See Block Diagram)
R
D
= 0
C
T
= .01 µF
C
T
= .02 µF
C
T
= .05 µF
C
T
= 0.1 µF
C
T
= 1 nF
C
T
= 5 nF
C
T
= 2 nF
6 5 7
R
T
R
D
C
T
200
100
50
20
10
5
2
− Timing Resistance − kR
T
Ω
1 2 5 10 20 50 100 200 1ms 2ms 5ms10ms
Charge Time − ms
500
400
300
200
100
0
0.2 0.5 1 2 5 10 20 50 100 200
− Dead Time Resistance −R
D
Ω
Charge Time − ms
C
T
= 1 nF
C
T
= 2 nF
C
T
= 5 nF
C
T
= .01 µF
C
T
= .05 µF
C
T
= 0.1 µF
C
T
= .02 µF
125°C
25°C
−55°C
Max R
D
For a Given R
T
,
Min R
T
For a Given R
D
500
400
300
200
100
0
2 4 6 8 10 12
Minimum Recommended R
T
− kW
Maximum recommended RD
R
L
= ∞
R
L
= 1 MΩ
R
L
= 300 kΩ
R
L
= 100 kΩ
R
L
= 30 kΩ
Voltage Gain
Phase
80
60
40
20
0
100 1 k 10 k 100 k 1 M 10 M
f − Frequency − Hz
Open-Loop Voltage Gain − dB
−360°
−270°
−180°
Open-Loop Phase
V
IN
= 20 V,
T
J
= 25°C
UC1525A, UC1527A
UC2525A, UC2527A
UC3525A, UC3527A
SLUS191C – FEBRUARY 1997 – REVISED JANUARY 2008
PRINCIPLES OF OPERATION AND TYPICAL CHARACTERISTICS (continued)
Since both the compensation and soft-start terminals (Pins 9 and 8) have current source pull-ups, either can
readily accept a pull-down signal which only has to sink a maximum of 100 A to turn off the outputs. This is
subject to the added requirement of discharging whatever external capacitance may be attached to these pins.
An alternate approach is the use of the shutdown circuitry of Pin 10 which has been improved to enhance the
available shutdown options. Activating this circuit by applying a positive signal on Pin 10 performs two functions;
the PWM latch is immediately set providing the fastest turn-off signal to the outputs; and a 150-A current sink
begins to discharge the external soft-start capacitor. If the shutdown command is short, the PWM signal is
terminated without significant discharge of the soft-start capacitor, thus, allowing, for example, a convenient
implementation of pulse-by-pulse current limiting. Holding Pin 10 high for a longer duration, however, will
ultimately discharge this external capacitor, recycling slow turn-on upon release.
Pin 10 should not be left floating as noise pickup could conceivably interrupt normal operation. All transitions of
the voltage on pin 10 should be within the time frame of one clock cycle and not repeated at a frequency higher
than 10 clock cycles.
Oscillator Charge Time vs R
T
and C
T
Oscillator Discharge Time vs R
T
C
T
Figure 8. Figure 9.
Maximum Value R
D
vs Minimum Value R
T
Error Amplifier Voltage Gain and Phase vs Frequency
Figure 10. Figure 11.
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