LM1575, LM2575-N, LM2575HV
SNVS106E –MAY 1999–REVISED APRIL 2013
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C
OUT
V
C
IN
4
1000 mF
LM2575-12
Output
1
Low ESR
2
3
GND
5
1N5817
+
IN
ON/OFF
100 mF
V
OUT
= -12V
150 mH
-V
IN
-5V to -12V
Typical Load Current
200 mA for VIN = −5.2V
500 mA for VIN = −7V
Pin numbers are for TO-220 package.
Figure 33. Negative Boost
UNDERVOLTAGE LOCKOUT
In some applications it is desirable to keep the regulator off until the input voltage reaches a certain threshold. An
undervoltage lockout circuit which accomplishes this task is shown in Figure 34, while Figure 35 shows the same
circuit applied to a buck-boost configuration. These circuits keep the regulator off until the input voltage reaches
a predetermined level.
VTH ≈ VZ1 + 2VBE (Q1)
(16)
DELAYED STARTUP
The ON /OFF pin can be used to provide a delayed startup feature as shown in Figure 36. With an input voltage
of 20V and for the part values shown, the circuit provides approximately 10 ms of delay time before the circuit
begins switching. Increasing the RC time constant can provide longer delay times. But excessively large RC time
constants can cause problems with input voltages that are high in 60 Hz or 120 Hz ripple, by coupling the ripple
into the ON /OFF pin.
ADJUSTABLE OUTPUT, LOW-RIPPLE
POWER SUPPLY
A 1A power supply that features an adjustable output voltage is shown in Figure 37. An additional L-C filter that
reduces the output ripple by a factor of 10 or more is included in this circuit.
22
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