July 2005

5

MIC5021

MIC5021

Micrel, Inc.

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

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

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

voltage. The boost capacitor charge is directed through

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 5µA out of

SENSE

steals base current from an internal drive transistor shutting

off the external MOSFET.

Overcurrent Limiting

a MOSFET Q1.

charged.

When the gate output turns the MOSFET off, the overcurrent

signal is removed from the sense inputs which deactivates

retry while the capacitor(s) recharge. Retry delay is increased

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

input is changed to TTL low.

Supply Voltage

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

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 volt-

age 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 com-

ponents 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.)