AH-29 [MICREL]

Protecting Super LDO⑩ Regulator MOSFETs by Jerry Kmetz; 保护超级LDO⑩稳压器的MOSFET由杰里Kmetz


型号：	AH-29
厂家：	MICREL SEMICONDUCTOR
描述：	Protecting Super LDO⑩ Regulator MOSFETs by Jerry Kmetz 保护超级LDO⑩稳压器的MOSFET由杰里Kmetz 稳压器
文件：	总2页 (文件大小：29K)
中文：	中文翻译
下载：	下载PDF数据表文档文件

AH-29

Micrel

Application Hint 29

Protecting Super LDO™ Regulator MOSFETs

by Jerry Kmetz

The Problem

control the enable-input pin of the regulator. The regulator

can be immediately disabled upon detection of a low-voltage

condition.

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

An Example

approximately V × I

output of the power supply is shorted it becomes

, or (5V – 3.3V) × 10A = 17W. If the

Figure 2 implements both the current-limit capability and a

control scheme for dealing with shorted outputs. The 2.3mΩ

OUT

(V / R

) × R

resistor R provides for current limiting at about 15A. The

DS(on)

current-limitthresholdvoltageoftheMIC5158isabout0.035V,

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

continuallymonitored;thesystemwillprotectthepassdevice

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.

or an unworkable

(5V / 0.028Ω) × 0.028Ω = 892W

Eventhemostconservativeheat-sinkdesignwillnotsavethe

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-

ablelevel, theadditionaldissipationimposedbytheshortcan

still damage the pass device. When considerable voltage is

being dropped by the pass device the short-circuit power

dissipation becomes dramatically high.

Circuit Description

Schmitt-trigger NAND gate A is used to control a gated

oscillator (gate B). Resistors R5 and R6, diode D3, and

capacitorC5provideoscillatortiming. Withthevaluesshown

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

The second feature offered by the MIC5156/7/8 parts is an

error flag. This is an open-collector output which generates a

signaliftheoutputvoltageisapproximately6%ormorelower

than the intended value. Since this flag output is asserted

logic low in the event of a shorted output, it can be used to

3.3µF

0.1µF

MIC5158

10 11 12 13 14

0.1µF

V_IN

V_OUT

(5V)

3.3V, 10A

17.8k

Q1*

IRLZ44

47µF

_OUT**

C_IN

47µF

10.7k

*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

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 (V_T+ and V_T–) 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.

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

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

3.3µF

0.1µF

MIC5158

10 11 12 13 14

C1 0.1µF

10k

R_S***

V_OUT

V_IN

3.3V, 10A

(5V)

2.3mΩ

17.8k

Q1*

IRLZ44

47µF

_OUT**

C_IN

47µF

10.7k

V_IN

10k

33k

CD4093BC

All Diodes Are 1N914

470pF

0.01µF

System

Enable

*For V_IN> 5V, use IRFZ44

**Improves transient response to load changes

0.035V

* **R_S

≈

I_LIMIT

Figure 2. Short-Circuit Protected 10A Regulator

1997

3-207

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