AH-27
Micrel
Application Hint 27
Slowing Voltage Regulator Turn-On
by Jerry Kmetz
Slow Turn-On Circuits
The turn-on time interval of a voltage regulator is essentially
determined by the bandwidth of the regulator, its maximum
output current, and the load capacitance. To some extent the
rise time of the applied input voltage (which is normally quite
short, tens of milliseconds, or less) also affects the turn-on
time. However, the regulator output voltage typically steps
abruptly at turn-on. Increasing the turn-on interval via some
form of slew-limiting decreases the surge current seen by
both the regulator and the system. This application hint
addresses designing circuitry to change the step-function to
a smoother RC charge waveform.
Various performance features exist between the three cir-
cuits that are presented. These are (1) whether stability is
impacted, (2) whether start-up output is 0V, and (3) whether
the circuit quickly recovers from momentarily interrupted
input voltage or shorted output. The following table summa-
rizes the features of each circuit:
Circuit
Stability
Start-Up V
IN
Interrupt
Number Impacted? Pedestal? Recovery?
1
2
3
yes
no
no
1.2V
1.8V
0V
no
no
yes
V
OUT
Short
Recovery?
no
yes
no
As C
T
charges, the regulator output (V
OUT
) asymptotically
approaches the desired value. If a turn-on time of 300
milliseconds is desired then about three time constants
should be allowed for charge time:
then 3τ = 0.3s, or
τ
= 0.1s = R1
×
C
T
= 300kΩ
×
0.33µF.
Figure 2 shows the waveforms of the circuit of Figure 1. This
circuit has three shortcomings: (1) the approximately 1.2V
step at turn-on, (2) the addition of capacitor C
T
places a zero
in the closed-loop transfer function (which affects frequency
and transient responses and can potentially cause stability
problems) and (3) the recovery time associated with a mo-
mentarily short-circuited output may be unacceptably long.
This is because if the output is shorted C
T
is discharged only
by R2; if the short is removed before C
T
is fully discharged the
regulator output will not exhibit the desired turn-on behavior.
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
10
5
0
Slow Turn-On Circuit Performance Features
1. The Simplest Approach
Figure 1 illustrates a typical LDO voltage regulator, the
MIC29153, with an additional capacitor (C
T
) in parallel with
the series leg (R1) of the feedback voltage divider. Since the
voltage (V
ADJ
) will be maintained at V
REF
by the regulator
loop, the output of this circuit will still rapidly step to V
REF
and
then rise slowly. Since V
REF
is usually only about 1.2V, this
eliminates a large part of the surge current.
4
2
0
0
0.2
0.4
0.6
0.8
1.0
TIME (s)
Figure 2. Turn-On Behavior for Circuit of Figure 1
Typical LDO Regulator
V
IN
IN
C
IN
22µF
GND
MIC29153
V
REF
ADJ
R2
100k
V
ADJ
OUT
R1
300k
C
T
0.33µF
C
OUT
22µF
V
OUT
Figure 1. Simplest Slow Turn-On Circuit
3-202
1997
MICREL [ MICREL SEMICONDUCTOR ]
