Datasheet MIC5021 Datasheet (MICREL)

MIC5021 Micrel

MIC5021

High-Speed High-Side MOSFET Driver

General Description

The MIC5021 high-side MOSFET driver is designed to oper­ate at frequencies up to 100kHz (5kHz PWM for 2% to 100%
duty cycle) and is an ideal choice for high speed applications
such as motor control, SMPS (switch mode power supplies),
and applications using IGBTs. The MIC5021 can also
operate as a circuit breaker with or without automatic retry.

A rising or falling edge on the input results in a current source
pulse or sink pulse on the gate output. This output current
pulse can turn on a 2000pF MOSFET in approximately
550ns. The MIC5021 then supplies a limited current (< 2mA),
if necessary, to maintain the output state.

An overcurrent comparator with a trip voltage of 50mV makes
the MIC5021 ideal for use with a current sensing MOSFET.
An external low value resistor may be used instead of a
sensing MOSFET for more precise overcurrent control. An
optional external capacitor placed from the CT pin to ground
may be used to control the current shutdown duty cycle (dead
time) from 20% to < 1%. A duty cycle from 20% to about 75%
is possible with an optional pull-up resistor from CT to VDD.

The MIC5021 is available in 8-pin SOIC and plastic DIP
packages.

Other members of the MIC502x family include the MIC5020
low-side driver and the MIC5022 half-bridge driver with a
cross-conduction interlock.

Features

• 12V to 36V operation

• 550ns rise/fall time driving 2000pF

• TTL compatible input with internal pull-down resistor

• Overcurrent limit

• Gate to source protection

• Internal charge pump

• 100kHz operation guaranteed over full temperature and
operating voltage range

• Compatible with current sensing MOSFETs

• Current source drive reduces EMI

Applications

• Lamp control

• Heater control

• Motor control

• Solenoid switching

• Switch-mode power supplies

• Circuit breaker

Ordering Information

Part Number Temperature Range Package

MIC5021BM –40°C to +85°C 8-pin SOIC
MIC5021BN –40°C to +85°C 8-pin Plastic DIP

5

Typical Application

TTL Input

10µF

optional*

+12V to +36V

MIC5021

1
2
3
4

V

DD

Input
C

T

Gnd

V

Sense
Sense

BOOST

Gate

8
7
6
5

N-Channel
Power MOSFET

2.7
nF

R

Load

SENSE

50mV

=

R

SENSE

* increases time before retry

I

TRIP

High-Side Driver with Overcurrent Trip and Retry

October 1998 5-169

MIC5021 Micrel

Pin Configuration

Block Diagram

Sense +
Sense –

Input

1

V

DD

Input

2

C

3

T

Gnd

4

DIP Package SOIC Package

6V Internal Regulator

I

1

C

INT

2I

1

50mV

V

BOOST

Gate
Sense−
Sense+

8
7
6

5

1

V

DD

2

Input
C

T

Gnd

Sense−
Sense+

3
4

(N) (M)

Fault

Normal

Q1

↑
↓

ONE-

SHOT

10I

2

V

BOOST

I

2

Gate

ON

OFF

8
7
6

5

6V

CHARGE

PUMP

15V

C

T

V

DD

V

BOOST

Gate

Transistor: 106

Pin Description

Pin Number Pin Name Pin Function

1V
2 Input TTL Compatible Input: Logic high turns the external MOSFET on. An

3C

4 Gnd Circuit Ground
5 Sense + Current Sense Comparator (+) Input: Connect to high side of sense resistor

6 Sense – Current Sense Comparator (–) Input: Connect to the low side of the sense

7 Gate Gate Drive: Drives the gate of an external power MOSFET. Also limits V

8V

DD

T

BOOST

Supply: +12V to +36V. Decouple with ≥ 10µF capacitor.

internal pull-down returns an open pin to logic low.
Retry Timing Capacitor: Controls the off time (t

retry cycle. (Duty cycle adjustment.)

) of the overcurrent

G(OFF)

• Open = approx. 20% duty cycle.

• Capacitor to Ground = approx. 20% to < 1% duty cycle.

• Pull-up resistor = approx. 20% to approx. 75% duty cycle.

• Ground = maintained shutdown upon overcurrent condition.

or current sensing MOSFET sense lead. A built-in offset in conjunction with
R

sets the load overcurrent trip point.

SENSE

resistor (usually the high side of the load).

to 15V max. to prevent Gate-to-Source damage. Will sink and source

GS

current.
Charge Pump Boost Capacitor: A bootstrap capacitor from V

FET source pin supplies charge to quickly enhance the Gate output during

BOOST

to the

turn-on.

5-170 October 1998

MIC5021 Micrel

Absolute Maximum Ratings

Supply Voltage (VDD) ..................................................+40V

Input Voltage ................................................ –0.5V to +15V

Sense Differential Voltage..........................................±6.5V

Sense + or Sense – to Gnd.......................... –0.5V to +36V

Operating Ratings

Supply Voltage (VDD) ....................................+12V to +36V

Temperature Range

PDIP....................................................... –40°C to +85°C

SOIC ...................................................... –40°C to +85°C

Timer Voltage (CT) .....................................................+5.5V

V

Capacitor .................................................... 0.01µF

BOOST

Electrical Characteristics

TA = 25°C, Gnd = 0V, VDD = 12V, CT = Open, Gate CL = 1500pF (IRF540 MOSFET) unless otherwise specified

Symbol Parameter Condition Min Typ Max Units

D.C. Supply Current VDD = 12V, Input = 0V 1.8 4 mA

VDD = 36V, Input = 0V 2.5 6 mA
VDD = 12V, Input = 5V 1.7 4 mA

VDD = 36V, Input = 5V 2.5 6 mA
Input Threshold 0.8 1.4 2.0 V
Input Hysteresis 0.1 V
Input Pull-Down Current Input = 5V 10 20 40 µA
Current Limit Threshold Note 1 30 50 70 mV
Gate On Voltage VDD = 12V Note 2 16 18 21 V

VDD = 36V Note 2 46 50 52 V

t

G(ON)

t

G(OFF)

t

DLH

t

R

t

DLH

t

F

f

max

Note 1 When using sense MOSFETs, it is recommended that R
Note 2 DC measurement.
Note 3 Input switched from 0.8V (TTL low) to 2.0V (TTL high), time for Gate transition from 0V to 2V.
Note 4 Input switched from 0.8V (TTL low) to 2.0V (TTL high), time for Gate transition from 2V to 17V.
Note 5 Input switched from 2.0V (TTL high) to 0.8V (TTL low), time for Gate transition from 20V (Gate on voltage) to 17V.
Note 6 Input switched from 2.0V (TTL high) to 0.8V (TTL low), time for Gate transition from 17V to 2V.
Note 7 Frequency where gate on voltage reduces to 17V with 50% input duty cycle.

Gate On Time, Fixed Sense Differential > 70mV 2 6 10 µs
Gate Off Time, Adjustable Sense Differential > 70mV, CT = 0pF 10 20 50 µs
Gate Turn-On Delay Note 3 500 1000 ns
Gate Rise Time Note 4 400 500 ns
Gate Turn-Off Delay Note 5 800 1500 ns
Gate Fall Time Note 6 400 500 ns
Maximum Operating Frequency Note 7 100 150 kHz

< 50Ω. Higher values may affect the sense MOSFET’s current transfer ratio.

SENSE

5

October 1998 5-171

MIC5021 Micrel

Typical Characteristics

Supply Current vs.

2.5

2.0

1.5

(mA)

1.0

SUPPLY

I

0.5

0.0

Supply Voltage

VIN = 0V

VIN = 5V

5 10152025303540

V

SUPPLY

(V)

Gate Turn-On Delay vs.

1000

950

900

(ns)

850

ON 10V

t

800

750

Supply Voltage

V

= V

GATE

SUPPLY

C

= 1500pF (IRCZ34)

L

INCLUDES PROPAGATION DELAY

5 10152025303540

V

C

BOOST

SUPPLY

= 0.01µF

(V)

+ 10V

Gate Voltage Change

vs. Supply Voltage

25

V

= V

GATE

–V

SUPPLY

(V)

GATE

V

GATE

20

15

10

5

0

5 10152025303540

V

SUPPLY

(V)

Gate Turn-On Delay vs.

Gate Capacitance

2.5
V

= V

GATE

V

SUPPLY

2.0

1.5

(µs)

ON

t

1.0

0.5

INCLUDES PROPAGATION DELAY

0.0

0

1x10

1x1011x1021x1031x1041x10

SUPPLY

= 12V

C

GATE

+ 4V

(pF)

Gate Turn-On Delay vs.

900

850

800

(ns)

750

ON 4V

t

700

650

Supply Voltage

V

=V

GATE

C

= 1500pF (IRCZ34)

L

C

INCLUDES PROPAGATION DELAY

5 10152025303540

V

SUPPLY

BOOST

SUPPLY

= 0.01µF

(V)

+ 4V

Gate Turn-Off Delay vs.

2000

1750

1500

(ns)

1250

OFF 4V

t

1000

5

750

Supply Voltage

V

=V

GATE

R

= 400

L

INCLUDES PROPAGATION DELAY

5 10152025303540

SUPPLY

C
(IRCZ34)

V

SUPPLY

+ 4V

GATE

= 1500pF

(V)

Overcurrent Retry Duty

Cycle vs. Timing Capacitance

25

20

15

10

NOTE:

, t

t

ON

OFF

INDEPENDENT

5

OF V

RETRY DUTY CYCLE (%)

SUPPLY

0

0.1 1 10 100 1000 10000

Input

Gate

Sense +, –

Differential

TIME

CT (pF)

TTL (H)

15V (max.)

V

SUPPLY

Source

50mV

tON = 5µs

= 12V

0V

0V

6µs

Input Current vs.

Input Voltage

V

= 12V

SUPPLY

0 5 10 15 20 25

TTL (H)

0V

Source

50mV

0V

VIN (V)

Input

Gate

Sense +, –

Differential

100

80

60

(µA)

IN

I

40

20

0

15V (max.)

Timing Diagram 1. Normal Operation

20µs

Input

Sense +, –

Differential

Gate

TTL (H)

15V (max.)

Source

50mV

Sense Threshold vs.

80

70

60

50

40

VOLTAGE (mV)

30

20

6µs

0V

0V

Temperature

-60 -30 0 30 60 90 120 150
TEMPERATURE (°C)

Timing Diagram 2. Fault Condition, CT = Open Timing Diagram 3. Fault Condition, CT = Grounded

5-172 October 1998

MIC5021 Micrel

Functional Description

Refer to the MIC5021 block diagram.

Input

A signal greater than 1.4V (nominal) applied to the MIC5021

INPUT causes gate enhancement on an external MOSFET

turning the MOSFET on.
An internal pull-down resistor insures that an open INPUT

remains low, keeping the external MOSFET turned off.

Gate Output

Rapid rise and fall times on the GATE output are possible
because each input state change triggers a one-shot which
activates a high-value current sink (10I2) for a short time. This
draws a high current though a current mirror circuit causing
the output transistors to quickly charge or discharge the
external MOSFET’s gate.

A second current sink continuously draws the lower value of
current used to maintain the gate voltage for the selected
state.

An internal charge pump utilizes an external “boost” capacitor
connected between V
MOSFET. (Refer to typical application.) The boost capacitor
stores charge when the MOSFET is off. As the MOSFET
turns on, its source to ground voltage increases and is added
to the voltage across the capacitor, raising the V
voltage. The boost capacitor charge is directed through the

GATE pin to quickly charge the MOSFET’s gate to 16V

maximum above VDD. The internal charge pump maintains
the gate voltage.

and the source of the external

BOOST

BOOST

pin

An internal zener diode protects the external MOSFET by
limiting the gate to source voltage.

Sense Inputs

The MIC5021’s 50mV (nominal) trip voltage is created by
internal current sources that force approximately 5µA out of

SENSE + and approximately 15µA (at trip) out of SENSE –.

When SENSE – is 50mV or more below SENSE +, SENSE –
steals base current from an internal drive transistor shutting
off the external MOSFET.

Overcurrent Limiting

Current source I1 charges C

upon power up. An optional

INT

external capacitor connected to CT is kept discharged through
a MOSFET Q1.

A fault condition (> 50mV from SENSE + to SENSE –) causes
the overcurrent comparator to enable current sink 2I1 which
overcomes current source I1 to discharge C
When C
off the gate output, and C

is discharged, the INPUT is disabled, which turns

INT

and CT are ready to be charged.

INT

in a short time.

INT

When the gate output turns the MOSFET off, the overcurrent
signal is removed from the sense inputs which deactivates
current sink 2I1. This allows C

and the optional capacitor

INT

connected to CT to recharge. A Schmitt trigger delays the
retry while the capacitor(s) recharge. Retry delay is in­creased by connecting a capacitor to CT (optional).

The retry cycle will continue until the fault is removed or the
input is changed to TTL low.

If CT is connected to ground, the circuit will not retry upon a
fault condition.

5

Applications Information

The MIC5021 MOSFET driver is intended for high-side
switching applications where overcurrent limiting and high
speed are required. The MIC5021 can control MOSFETs that
switch voltages up to 36V.

High-Side Switch Circuit Advantages

High-side switching allows more of the load related compo­nents and wiring to remain near ground potential when
compared to low-side switching. This reduces the chances
of short-to-ground accidents or failures.

Speed Advantage

The MIC5021 is about two orders of magnitude faster than
the low cost MIC5014 making it suitable for high-frequency
high-efficiency circuit operation in PWM (pulse width modu­lation) designs used for motor control, SMPS (switch mode
power supply) and heating element control.

Switched loads (on/off) benefit from the MIC5021’s fast
switching times by allowing use of MOSFETs with smaller
safe operating areas. (Larger MOSFETs are often required
when using slower drivers.)

Supply Voltage

The MIC5021’s supply input (VDD) is rated up to 36V. The
supply voltage must be equal to or greater than the voltage
applied to the drain of the external N-channel MOSFET.

A 16V minimum supply is recommended to produce continu­ous on-state, gate drive voltage for standard MOSFETs (10V
nominal gate enhancement).

When the driver is powered from a 12V to 16V supply, a logic­level MOSFET is recommended (5V nominal gate enhance­ment).

PWM operation may produce satisfactory gate enhancement
at lower supply voltages. This occurs when fast switching
repetition makes the boost capacitor a more significant
voltage supply than the internal charge pump.

October 1998 5-173

MIC5021 Micrel

Logic-Level MOSFET Precautions

Logic-level MOSFETs have lower maximum gate-to-source
voltage ratings (typically ±10V) than standard MOSFETs
(typically ±20V). When an external MOSFET is turned on, the
doubling effect of the boost capacitor can cause the gate-to­source voltage to momentarily exceed 10V. Internal zener
diodes clamp this voltage to 16V maximum which is too high
for logic-level MOSFETs. To protect logic-level MOSFETs,
connect a zener diode (5V≤V

<10V) from gate to source.

Zener

Overcurrent Limiting

A 50mV comparator is provided for current sensing. The low
level trip point minimizes I2R losses when a power resistor is
used for current sensing.

The adjustable retry feature can be used to handle loads with
high initial currents, such as lamps or heating elements, and
can be adjusted from the CT connection.

CT to ground maintains gate drive shutdown following an
overcurrent condition.

CT open, or a capacitor to ground, causes automatic retry.
The default duty cycle (CT open) is approximately 20%. Refer
to the electrical characteristics when selecting a capacitor for
reduced duty cycle.

CT through a pull-up resistor to VDD increases the duty cycle.

Increasing the duty cycle increases the power dissipation in
the load and MOSFET under a “fault” condition.

Circuits may
become unstable at a duty cycle of about 75% or higher,
depending on conditions.

Caution: The MIC5021 may be
damaged if the voltage applied to CT exceeds the absolute
maximum voltage rating.

Boost Capacitor Selection

The boost capacitor value will vary depending on the supply
voltage range.

+12V to +20V

MIC5021

10µF

TTL Input

1

V

DD

2

Input

3

C

T

4

Gnd

V

BOOST

Gate
Sense
Sense

8
7
6

0.01
µF

5

Load

A 0.01µF boost capacitor is recommended for best perfor­mance in the 12V to 20V range. Refer to figure 1. Larger
capacitors may damage the MIC5021.

+12V to +36V

MIC5021

10µF

TTL Input

1

V

DD

2

Input

3

C

T

4

Gnd

V

BOOST

Gate
Sense
Sense

8
7
6

2.7
nF

5

Load

Figure 2. 12V to 36V Configuration

If the full 12V to 36V voltage range is required, the boost
capacitor value must be reduced to 2.7nF. Refer to Figure 2.
The recommended configuration for the 20V to 36V range is
to place the capacitor is placed between VDD and V

BOOST

as

shown in Figure 3.

+12V to +36V

0.1
µF

8
7
6
5

Load

10µF

TTL Input

1
2
3
4

V

DD

Input
C

T

Gnd

MIC5021

V

BOOST

Sense
Sense

Gate

Figure 3. Preferred 20V to 36V Configuration

Do not use both boost capacitor between V
MOSFET source and V

and VDD at the same time.

BOOST

BOOST

and the

Current Sense Resistors

Lead length can be significant when using low value (< 1Ω)
resistors for current sensing. Errors caused by lead length
can be avoided by using four-teminal current sensing resis­tors. Four-terminal resistors are available from several
manufacturers.

Figure 1. 12V to 20V Configuration

5-174 October 1998

MIC5021 Micrel

V

DD

Input
C

T

Gnd

V

BOOST

Gate
Sense
Sense

TTL Input

R

SENSE

N-Channel
Power MOSFET
(IRF540)

+20V to +36V

MIC5021

1
2
3
4

8
7
6
5

10µF

Solenoid

(24V, 47Ω)

0.01
µF

Schottky
Diode
(1N5822)

(+24V)

(< 0.08Ω)

Circuits Without Current Sensing

V+

MIC5021

TTL Input

10µF

1

V

DD

2

Input

3

C

T

4

Gnd

V

BOOST

Gate
Sense−
Sense+

8
7
6
5

N-Channel
Power MOSFET

0.01
µF

Load

Figure 4a. Connecting Sense to Source

V+

MIC5021

TTL Input

10µF

1

V

DD

2

Input

3

C

T

4

Gnd

V

BOOST

Gate
Sense−
Sense+

8
7
6
5

N-Channel
Power MOSFET

0.01
µF

Load

Figure 4b. Connecting Sense to Supply

Current sensing may be omitted by connecting the SENSE +
and SENSE – pins to the source of the MOSFET or to the
supply. Connecting the SENSE pins to the supply is preferred
for inductive loads. Do not connect the SENSE pins to ground.

Inductive Load Precautions

Circuits controlling inductive loads, such as solenoids (Figure

5) and motors, require precautions when controlled by the
MIC5021. Wire wound resistors, which are sometimes used
to simulate other loads, can also show significant inductive
properties.

An inductive load releases stored energy when its current
flow is interrupted (when the MOSFET is switched off). The
voltage across the inductor reverses and the inductor at­tempts to force current flow. Since the circuit appears open
(the MOSFET appears as a very high resistance) a very large
negative voltage occurs across the inductor.

Limiting Inductive Spikes

The voltage across the inductor can be limited by connecting
a Schottky diode across the load. The diode is forward biased
only when the load is switched off. The Schottky diode
clamps negative transients to a few volts. This protects the
MOSFET from drain-to-source breakdown and prevents the
transient from damaging the charge pump by way of the boost
capacitor. Also see

Sense Pin Considerations

below.

The diode should have a peak forward current rating greater
than the load current. This is because the current through the
diode is the same as the load current at the instant the
MOSFET is turned off.

Figure 5. Solenoid Driver

with Current Sensing

Sense Pin Considerations

The sense pins of the MIC5021 are sensitive to negative
voltages. Forcing the sense pins much below –0.5V effec­tively reverses the supply voltage on portions of the driver
resulting in unpredictable operation or damage.

MIC5021

1

V

DD

2

Input

3

C

T

4

Forward drop across diodes
allows leads to go negative.

Current flows from ground (0V)
through the diodes to the load
during negative transcients.

Gate

8
7
6
5

MOSFET

Turnoff

Inductive
Load

~V

DD

0V

Negative
Spike

Figure 6. Inductive Load Turnoff

Figure 6 shows current flowing out of the sense leads of an
MIC5021 during a negative transient (inductive kick). Internal
Schottky diodes attempt to limit the negative transient by
maintaining a low forward drop.

Although the internal Schottky diodes can protect the driver
in low-current resistive applications, they are inadequate for
inductive loads or the lead inductance in high-current resis­tive loads. Because of their small size, the diodes’ forward
voltage drop quickly exceeds 0.5V as current increases.

5

October 1998 5-175

MIC5021 Micrel

V

DD

Input
C

T

Gnd

V

BOOST

Gate
Sense
Sense

TTL Input

Wirewound
Resistor
(3Ω)

N-Channel
Power MOSFET
(IRFZ44)

+12V to +20V

R

SENSE

(< 0.01Ω)

(+12V)

MIC5021

1
2
3
4

8
7
6
5

10µF

0.01
µF

External Protection

Resistors placed in series with each SENSE connection limit
the current drawn from the internal Schottky diodes during a
negative transient. This minimizes the forward drop across
the diodes.

MIC5021

1

V

DD

2

Input

3

C

T

4

Gnd

VR1 = V

R2

to avoid skewing
the 50mV trip point.

(5mV suggested)

R1

≅

3 × R2

V

BOOST

Gate
Sense−
Sense+

8
7
6
5

R1

5µA

V

R1

R2

15µA

V

.

R2

N-Channel
Power MOSFET

50mV nominal

R

S

(at trip)

Load

Figure 7. Resistor Voltage Drop

During normal operation, sensing current from the sense pins
is unequal (5µA and 15µA). The internal Schottky diodes are
reverse biased and have no effect. To avoid skewing the trip
voltage, the current limiting resistors must drop equal volt­ages at the trip point currents. See Figure 7. To minimize
resistor tolerance error, use a voltage drop lower than the trip
voltage of 50mV. 5mV is suggested.

External Schottky diodes are also recommended. See D2
and D3 in Figure 8. The external diodes clamp negative
transients better than the internal diodes because their larger
size minimizes the forward voltage drop at higher currents.

+12V to +36V

MIC5021

10µF

TTL Input

1

V

DD

2

Input

3

C

T

4

Gnd

V

BOOST

Gate
Sense
Sense

11DQ03

8
7
6
5

D2

D3

11DQ03

2.7
nF

R1

1.0k

R2

330Ω

D1

N-Channel
Power MOSFET

R

SENSE

Inductive
Load

Figure 8. Protection from Inductive Kick

High-Side Sensing

Sensing the current on the high side of the MOSFET isolates
the SENSE pins from the inductive spike.

Figure 9. High Side Sensing

Lamp Driver Application

Incandescent lamps have a high inrush current (low resis­tance) when turned on. The MIC5021 can perform a “soft
start” by pulsing the MOSFET (overcurrent condition) until
the filament is warm and its current decreases (resistance
increases). The sense resistor value is selected so the
voltage drop across the sense resistor decreases below the
sense threshold (50mV) as the filament becomes warm. The
FET is no longer pulsed and the lamp turns completely on.

V+

(+12V)

MIC5021

TTL Input

10µF

"( )" values apply to demo circuit.

1

V

DD

2

Input

3

C

T

4

Gnd

See text.

V

BOOST

Gate
Sense−
Sense+

8
7
6
5

N-Channel
Power MOSFET
(IRF540)

0.01
µF

R
(0.041Ω)

SENSE

Incandescent
Lamp (#1157)

Figure 10. 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 10 allows
for soft start of the higher-resistance filament (measures
approx. 2.1Ω cold or 21Ω hot).

5-176 October 1998

MIC5021 Micrel

V

DD

Input
C

T

Gnd

V

BOOST

Gate
Sense
Sense

TTL Input

R

SENSE

+12V to +36V

MIC5021AJB

1
2
3
4

8
7
6
5

10µF

2.7
nF

Load

2.2M

add resistor for

–40

°

C to –55°C

operation

Remote Overcurrent Limiting Reset

In circuit breaker applications where the MIC5021 maintains
an off condition after an overcurrent condition is sensed, the
CT pin can be used to reset the MIC5021.

+12V to +20V

MIC5021

TTL Input

74HC04

(example)

Retry (H)

Maintained (L)

10k to

100k

10µF

2N3904
Q1

1
2
3
4

V

DD

Input
C

T

Gnd

V

BOOST

Gate
Sense
Sense

8
7
6
5

0.01
µF

N-Channel
Power
MOSFET

R

SENSE

Load

Figure 11. Remote Control Circuit

Switching Q1 on pulls CT low which keeps the MIC5021 GATE
output off when an overcurrent is sensed. Switching Q1 off
causes CT to appear open. The MIC5021 retries in about
20µs and continues to retry until the overcurrent condition is
removed.

For demonstration purposes, a 680Ω load resistor and 3Ω
sense resistor will produce an overcurrent condition when the
load’s supply (V+) is approximately 12V or greater.

Low-Temperature Operation

As the temperature of the MIC5021AJB (extended tempera­ture range version—no longer available) approaches –55°C,
the driver’s off-state, gate-output offset from ground in­creases. If the operating environment of the MIC5021AJB
includes low temperatures (–40°C to –55°C), add an external

2.2MΩ resistor as shown in Figures 12a or 12b. This assures
that the driver’s gate-to-source voltage is far below the
external MOSFET’s gate threshold voltage, forcing the
MOSFET fully off.

+12V to +36V

MIC5021AJB

TTL Input

10µF

1

V

DD

2

Input

3

C

T

4

Gnd

add resistor for

–40

°

C to –55°C

operation

V

BOOST

Gate
Sense
Sense

8
7
6

2.7

2.2M

nF

5

R

SENSE

Load

Figure 12a. Gate-to-Source Pull Down

The gate-to-source configuration (refer to Figure 12a) is
appropriate for resistive and inductive loads. This also
causes the smallest decrease in gate output voltage.

Figure 12b. Gate-to-Ground Pull Down

The gate-to-ground configuration (refer to Figure 12b) is
appropriate for resistive, inductive, or capacitive loads. This
configuration will decrease the gate output voltage slightly
more than the circuit shown in Figure 12a.

5

October 1998 5-177