AH-29
Micrel
Application Hint 29
Protecting Super LDO™ Regulator MOSFETs
by Jerry Kmetz
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 / 0.028Ω)
2
×
0.028Ω = 892W
Even the most conservative heat-sink design will not save the
MOSFET.
The Micrel MIC5156, MIC5157, and MIC5158 Super LDO™
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
C2
0.1µF
7
C2+
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 2.3mΩ
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 110µs 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
C3
3.3µF
6
C2–
5
V
CP
4
GND
3
FLAG
2
5V FB
S
EA
1
MIC5158
C1+
C1–
V
DD
G
D
EN
8
C1
0.1µF
V
IN
(5V)
C
IN
47µF
9
10
11 12 13
14
Q1*
IRLZ44
C
OUT
**
47µF
R1
17.8k
1%
R2
10.7k
1%
V
OUT
3.3V, 10A
*For V
IN
> 5V, use IRFZ44
* *Improves transient response to load changes
Figure 1. Simple 10A, 5V to 3.3V, Voltage Regulator
3-206
1997
MICREL [ MICREL SEMICONDUCTOR ]
