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LM2595
Delayed Startup
There are some applications, like the inverting regulator
already mentioned above, which require a higher amount of
startup current. In such cases, if the input power source is
+V in
+V in
2
LM2595
limited, this delayed startup feature becomes very useful.
To provide a time delay between the time when the input
voltage is applied and the time when the output voltage
R2
10 k
R3
47 k
C in
100 m F 5
ON/OFF 3
GND
comes up, the circuit in Figure 28 can be used. As the input
voltage is applied, the capacitor C1 charges up, and the
voltage across the resistor R2 falls down. When the voltage
Z1
1N5242B
on the ON/OFF pin falls below the threshold value 1.3 V, the
regulator starts up. Resistor R1 is included to limit the
maximum voltage applied to the ON/OFF pin. It reduces the
power supply noise sensitivity, and also limits the capacitor
R1
10 k
Q1
2N3904
V th ≈ 13 V
C1 discharge current, but its use is not mandatory.
When a high 50 Hz or 60 Hz (100 Hz or 120 Hz
respectively) ripple voltage exists, a long delay time can
cause some problems by coupling the ripple into the
ON/OFF pin, the regulator could be switched periodically
on and off with the line (or double) frequency.
NOTE: This picture does not show the complete circuit.
Figure 29. Undervoltage Lockout Circuit for
Buck Converter
+V in
+V in
7
LM2595
+V in
+V in
2
LM2595
C in
100 m F
C1
0.1 m F
R1
47 k
5
ON/OFF 6
R2
47 k
GN
D
R2
15 k
Z1
1N5242B
R3
47 k
Q1
C in
100 m F 5
ON/OFF 3
GND
V th ≈ 13 V
2N3904
R1
NOTE: This picture does not show the complete circuit.
15 k
V out
Figure 28. Delayed Startup Circuitry
Undervoltage Lockout
Some applications require the regulator to remain off until
the input voltage reaches a certain threshold level. Figure 29
shows an undervoltage lockout circuit applied to a buck
regulator. A version of this circuit for buck ? boost converter
is shown in Figure 30. Resistor R3 pulls the ON/OFF pin
high and keeps the regulator off until the input voltage
reaches a predetermined threshold level with respect to the
ground Pin 3, which is determined by the following
expression:
NOTE: This picture does not show the complete circuit.
Figure 30. Undervoltage Lockout Circuit for
Buck ? Boost Converter
Adjustable Output, Low ? Ripple Power Supply
A 1.0 A output current capability power supply that
features an adjustable output voltage is shown in Figure 31.
This regulator delivers 1.0 A into 1.2 V to 35 V output.
The input voltage ranges from roughly 3.0 V to 40 V. In order
to achieve a 10 or more times reduction of output ripple, an
additional L ? C filter is included in this circuit.
) 1.0 ) R2 V
V
th
[ V
Z1
R1
BE
(Q1)
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