AH-29
Micrel
3-206
1997
Application Hint 29
Protecting Super LDOTM Regulator MOSFETs
by Jerry Kmetz
control the enable-input pin of the regulator. The regulator
can be immediately disabled upon detection of a low-voltage
condition.
An Example
Figure 2 implements both the current-limit capability and a
control scheme for dealing with shorted outputs. The 2.3m
resistor R
S
provides for current limiting at about 15A. The
current-limit threshold voltage of the MIC5158 is about 0.035V,
and 0.035V/15A = 2.3mV. See Application Hint 25 for
information on building such resistors using circuit board
copper. Since a shorted output may be momentary, the
circuitry built around U1 automatically restarts the regulator
when a short is removed. Existence of a shorted output is
continually monitored; the system will protect the pass device
for an indefinite time. When a short exists the regulator is
enabled for a very brief interval and disabled for a much
longer interval; power dissipation is reduced by this duty
cycle, which can be arbitrarily designed.
Circuit Description
Schmitt-trigger NAND gate A is used to control a gated
oscillator (gate B). Resistors R5 and R6, diode D3, and
capacitor C5 provide oscillator timing. With the values shown
the enable time is about 110
s approximately every 2.25ms.
This provides a healthy 1:20 on/off ratio (5% duty cycle) for
reducing power dissipated by the pass device. Diode D2
keeps C5 discharged until gate A enables the oscillator. This
assures that oscillation will begin with a full-width short
enable pulse. Different enable and/or disable time(s) may be
The Problem
A momentary short can increase power dissipation in a
MOSFET voltage regulator pass device to a catastrophic
level. In the circuit of Figure 1, power dissipation in Q1 is
approximately V
DS
I
OUT
, or (5V 3.3V)
10A = 17W. If the
output of the power supply is shorted it becomes
(V
IN
/ R
DS(on)
)
2
R
DS(on)
,
or an unworkable
(5V / 0.028
)
2
0.028
= 892W
Even the most conservative heat-sink design will not save the
MOSFET.
The Micrel MIC5156, MIC5157, and MIC5158 Super LDOTM
Regulator Controllers offer two features that can be used to
save the pass device. The first feature is a current limit
capability (not implemented in Figure 1). Output current can
be limited at a user-defined value, but the function is not the
classic foldback scheme. While fixed-value current limiting
can reduce shorted-output power dissipation to a manage-
able level, the additional dissipation imposed by the short can
still damage the pass device. When considerable voltage is
being dropped by the pass device the short-circuit power
dissipation becomes dramatically high.
The second feature offered by the MIC5156/7/8 parts is an
error flag. This is an open-collector output which generates a
signal if the output voltage is approximately 6% or more lower
than the intended value. Since this flag output is asserted
logic low in the event of a shorted output, it can be used to
1
2
3
4
14
13
12
11
5
6
7
10
9
8
MIC5158
C2+
V
CP
GND
FLAG
5V FB
EA
C1+
C1
V
DD
G
D
S
EN
V
OUT
3.3V, 10A
V
IN
(5V)
C2
0.1F
C1
0.1F
C
OUT
**
47F
*For V
IN
> 5V, use IRFZ44
* *Improves transient response to load changes
C
IN
47F
C3
3.3F
R1
17.8k
1%
R2
10.7k
1%
Q1*
IRLZ44
C2
Figure 1. Simple 10A, 5V to 3.3V, Voltage Regulator
AH-29
Micrel
3
1997
3-207
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 (V
T
+ and V
T
) of Schmitt-trigger gate B.
Values for k1 and k2 (empirically derived from a breadboard)
are 0.33 and 0.23, respectively. Component tolerances were
ignored.
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
V
IN
if not required. Since the output of gate B is logic high
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
supply output may or may not be shorted, it is desirable to wait
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
to gate A) for about 4ms, to attempt to span the width of the
logic-low flag that is generated during a normal (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.
After start-up the logic-high inputs to gate A hold the oscillator
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.
1
2
3
4
14
13
12
11
5
6
7
10
9
8
MIC5158
C2+
V
CP
GND
FLAG
5V FB
EA
C1+
C1
V
DD
G
D
S
EN
V
OUT
3.3V, 10A
V
IN
(5V)
C2
0.1F
C1 0.1F
C
OUT
**
47F
*For V
IN
> 5V, use IRFZ44
* *Improves transient response to load changes
C
IN
47F
C3
3.3F
R1
17.8k
1%
R2
10.7k
1%
Q1*
IRLZ44
C2
D
11
14
V
IN
12
13
10
C
7
9
8
4
B
C5
0.01F
6
5
3
D2
D1
D3
R5
33k
R6
1M
R4
10k
1
2
A
C4
470pF
System
Enable
All Diodes Are 1N914
U1
CD4093BC
R3
10k
R
S
***
2.3m
* * *R
S
0.035V
I
LIMIT
Figure 2. Short-Circuit Protected 10A Regulator
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