October 1998

5-173

MIC5021

Micrel

5

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.

remains low, keeping the external MOSFET turned off.

Gate Output

because each input state change triggers a one-shot which

activates a high-value current sink (10I

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 V

the gate voltage.

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 5

µA out of

SENSE

+ and approximately 15

steals base current from an internal drive transistor shutting

off the external MOSFET.

Overcurrent Limiting

Current source I

external capacitor connected to C

a MOSFET Q1.

the overcurrent comparator to enable current sink 2I

overcomes current source I

When C

off the gate output, and C

When the gate output turns the MOSFET off, the overcurrent

signal is removed from the sense inputs which deactivates

current sink 2I

connected to C

retry while the capacitor(s) recharge.

Retry delay is in-

creased by connecting a capacitor to C

The retry cycle will continue until the fault is removed or the

input is changed to TTL low.

If C

fault condition.

Supply Voltage

The MIC5021’s supply input (V

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 continu-

ous on-state, gate drive voltage for standard MOSFETs (10V

nominal gate enhancement).

When the driver is powered from a 12V to 16V supply, a logic-

level MOSFET is recommended (5V nominal gate enhance-

ment).

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.

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 compo-

nents 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 modu-

lation) 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.)