MIC5020 Просмотр технического описания (PDF) - Micrel

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MIC5020

Current-Sensing Low-Side MOSFET Driver

Micrel 

MIC5020

Lamp Driver Application

Incandescent lamps have a high inrush current (low resis-

tance) when turned on. The MIC5020 can perform a “soft

start” by pulsing the MOSFET (overcurrent condition) until

the filament is warm enough for its current to decrease (re-

sistance increases). The sense resistor is selected so the

voltage across the sense resistor drops below the sense

threshold (50mV) as the filament becomes warm. The

MOSFET is no longer pulsed to limit current and the lamp

turns completely on.

V+

(+11V to +12V)

10µF

TTL Input

(0V/5V)

MIC5020

1

8

VDD

Gate

2

7

Input Sense-

3

6

Fault Sense+

4

CT

5

Gnd

“( )” values apply to

demo circuit. See text.

Incandescent

Lamp (#1157)

N-Channel

Power MOSFET

(IRF540)

RS E NS E

(0.041Ω)

Figure 1. Lamp Driver with

Current Sensing

A lamp may not fully turn on if the filament does not heat up

adequately. Changing the duty cycle, sense resistor, or both to

match the filament characteristics can correct the problem.

Soft start can be demonstrated using a #1157 dual-filament

automotive lamp. The value of RS shown in figure 1 allows

for soft start of the higher-resistance filament (measures

approx. 2.1Ω cold or 21Ω hot).

Solenoid Driver Application

The MIC5020 can be directly powered by the control voltage

supply in typical 11Vdc through 50Vdc control applications.

Current sensing has been omitted as an example.

V+

+11V to +50V

Solenoid

Diode

10µF

TTL Input

MIC5020

1

8

VDD

Gate

2

7

Input Sense-

3

6

Fault Sense+

4

CT

5

Gnd

N-Channel

Power MOSFET

Figure 2. Solenoid Driver,

Without Current Sensing

A diode across the load protects the MOSFET from the volt-

age spike generated by the inductive load upon MOSFET

turn off. The peak forward current rating of the diode should

be greater than the load current.

Micrel, Inc.

Current Sensing MOSFET Application

A current sensing MOSFET allows current sensing without

adding additional resistance to the power switching circuit.

A current sensing MOSFET has two source connections: a

“power source” for power switching and a “current source”

for current sensing. The current from the current source is

approximately proportional to the current through the power

source, but much smaller. A current sensing ratio (ISOURCE/

ISENSE) is provided by the MOSFET manufacturer.

V+

(+13.2V, > 4.4A)

+11V to +50V

(+13.2V)

10µF

TTL Input

(0V/5V)

MIC5020

1

8

VDD

Gate

2

7

Input Sense-

3

6

Fault Sense+

4

CT

5

Gnd

“( )” values apply to

demo circuit. See text.

(3Ω, > 60W)

N-Channel

Current Sensing

Power MOSFET

(IRCZ24)

RSENSE

(10Ω)

Figure 3. Using a Current Sensing MOSFET

The MOSFET current source is used to develop a voltage

across a sense resistor. This voltage is monitored by the

MIC5020 (SENSE + and SENSE – pins) to identify an over-

current condition.

The value of the sense resistor can be estimated with:

RSENSE = (r VTRIP RDS(ON)) / (ILOAD RDS(ON) – VTRIP)

where:

RSENSE = external “sense” resistor

VTRIP = 50mV (0.050V) for the MIC5020

r = manufacturer’s current sense ratio: (ISOURCE/ISENSE)

RDS(ON) = manufacturer’s power source on resistance

ILOAD = load current (power source current)

The drain to source voltage under different fault conditions

affects the behavior of the MOSFET current source; that is, the

current source will respond differently to a slight over-current

condition (VDS(ON) very small) than to a short circuit (where

VDS(ON) is approximately equal to the supply voltage).

Adjustment of the sense resistor value by experiment starting

from the above formula will provide the quickest selection

of RSENSE.

Refer to manufacture’s data sheets and application notes

for detailed information on current sensing MOSFET char-

acteristics.

Figure 3 includes values which can be used to demonstrate

circuit operation. The IRCZ24 MOSFET has a typical sense

ratio of 780 and a RDS(ON) of 0.10Ω. A large 3Ω wirewound

load resistor will cause inductive spikes which should be

suppressed using a diode (using the same configuration as

figure 2).

MIC5020

6

July 2005

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