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| 型号： | ah-29 |
| 厂商： | Micrel Semiconductor,Inc. |
| 元件分类： | 圆形连接器 |
| 英文描述： | Circular Connector; No. of Contacts:22; Series:MS27473; Body Material:Aluminum; Connecting Termination:Crimp; Connector Shell Size:12; Circular Contact Gender:Socket; Circular Shell Style:Straight Plug; Insert Arrangement:12-22 RoHS Compliant: No |
| 中文描述： | ⑩保护超级稳压器稳压器的MOSFET的杰里克梅茨 |
| 文件页数： | 1/2页 |
| 文件大小： | 25K |
| 代理商： | AH-29 |
AH-29
Micrel
3-206
1997
Application Hint 29
Protecting Super LDO 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
μ
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 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
1
2
3
4
14
13
12
11
5
6
7
10
9
8
MIC5158
C
V
C
G
F
5
E
C
C
V
D
G
D
S
E
V
3.3V, 10A
V
IN
(5V)
C2
0.1μF
C1
0.1μF
C
OUT
**
47μF
*For V
> 5V, use IRFZ44
**Improves transient response to load changes
C
IN
47μF
C3
3.3μF
R1
17.8k
1%
R2
10.7k
1%
Q1*
IRLZ44
C
Figure 1. Simple 10A, 5V to 3.3V, Voltage Regulator
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| AH0014CD | DIP Socket; No. of Contacts:64; Pitch Spacing:0.07"; Row Spacing:0.8"; Terminal Type:PC Board; Leaded Process Compatible:Yes; Peak Reflow Compatible (260 C):No RoHS Compliant: Yes |
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