AH-29
Micrel
appropriate for some applications. Enable time is approxi-
mately k1×R5×C5; disable time is approximately k2×R6×C5.
Constants k1 and k2 are determined primarily by the two
threshold voltages (VT+ and VT–) of Schmitt-trigger gate B.
Values for k1 and k2 (empirically derived from a breadboard)
are0.33and0.23, respectively. Componenttoleranceswere
ignored.
to gate A) for about 4ms, to attempt to span the width of the
logic-lowflagthatisgeneratedduringanormal(non-shorted)
regulator start-up. Providing enough delay time to span the
time of the flag may not always be practical, especially when
starting with high-capacitance loads. If the logic-low flag is
longer than the delayed enable input to gate A, the oscillator
will cycle through its on/off duty cycle and the circuit will again
attempt a normal start-up. This will result in a slowing of the
regulator turn-on, but this is not usually objectionable be-
cause it reduces turn-on surge currents.
Getting Started
The MIC5158 produces a brief logic-low error-flag output at
start-up because when first enabled the output voltage is
zero. Notice that the protection circuitry provides a system
enable input. Use of this input is optional; it should be tied to
Afterstart-upthelogic-highinputstogateAholdtheoscillator
off and the system remains enabled as long as no error flag
is generated. If the flag is generated due to a short, the
MIC5158 remains enabled only for the time of the oscillator
enable pulse and is then immediately disabled for the dura-
tion of the oscillator cycle. As long as the short exists the
oscillator runs and the system monitors the flag to detect
removal of the short; meanwhile the MOSFET is protected,
and the system again starts when the short is removed.
V
if not required. Since the output of gate B is logic high
IN
when the oscillator is disabled, a logic-high system enable
input enables the MIC5158, which immediately produces the
brief logic-low flag output mentioned above. Since the power
supplyoutputmayormaynotbeshorted,itisdesirabletowait
and see. The required wait-delay timing is implemented by
Resistor R4, capacitor C4, and diode D1. The leading-edge
of the regulator enable signal is delayed (before application
C3
3.3µF
C2
3
0.1µF
7
6
5
4
3
2
1
MIC5158
8
9
10 11 12 13 14
C1 0.1µF
R3
10k
RS***
VOUT
VIN
3.3V, 10A
(5V)
2.3mΩ
R1
17.8k
1%
Q1*
IRLZ44
C
47µF
OUT**
CIN
47µF
R2
10.7k
1%
VIN
14
12
13
R4
10k
11
D3
R5
33k
R6
1M
D
D1
U1
CD4093BC
D2
1
All Diodes Are 1N914
5
6
4
8
3
10
2
A
C4
470pF
9
C5
0.01µF
B
7
C
System
Enable
*For VIN > 5V, use IRFZ44
**Improves transient response to load changes
0.035V
* **RS
≈
ILIMIT
Figure 2. Short-Circuit Protected 10A Regulator
1997
3-207
MICREL [ MICREL SEMICONDUCTOR ]
