MIC5012

Micrel

5-122

April 1998

Gate Control Circuit

When applying the MIC5012, it is helpful to understand the

operation of the gate control circuitry (see Figure 12). The

gate circuitry can be divided into two sections: 1) charge

pump (oscillator, Q1-Q5, and the capacitors) and 2) gate

turn-off switch (Q6).

When the MIC5012 is in the OFF state, the oscillator is

turned off, thereby disabling the charge pump. Q5 is also

turned off, and Q6 is turned on. Q6 holds the gate pin (G) at

ground potential which effectively turns the external MOS-

FET off.

Q6 is turned off when the MIC5012 is commanded on, and

Q5 pulls the gate up to supply (through 2 diodes). Next, the

charge pump begins supplying current to the gate. The gate

accepts charge until the gate-source voltage reaches 12.5V

and is clamped by the zener diode.

A 2-output, three-phase clock switches Q1-Q4, providing a

quasi-tripling action. During the initial phase Q4 and Q2 are

ON. C1 is discharged, and C2 is charged to supply through

Q5. For the second phase Q4 turns off and Q3 turns on,

pushing C2 above supply (charge is dumped into the gate).

Q3 also charges C1. On the third phase Q2 turns off and Q1

turns on, pushing the common point of the two capacitors

above supply. Some of the charge in C1 makes its way to

the gate. The sequence is repeated by turning Q2 and Q4

back on, and Q1 and Q3 off.

In a low-side application operating on a 12 to 15V supply,

the MOSFET is fully enhanced by the action of Q5 alone. On

supplies of more than approximately 14V, current flows

directly from Q5 through the zener diode to ground. To

prevent excessive current flow, the MIC5012 supply should

be limited to 15V in low-side applications.

The action of Q5 makes the MIC5012 operate quickly in

low-side applications. In high-side applications Q5

precharges the MOSFET gate to supply, leaving the charge

pump to carry the gate up to full enhancement 10V above

supply. Bootstrapped high-side drivers are as fast as low-

side drivers since the chip supply is boosted well above the

drain at turn-on.

V

+

C1

C2

500

Ω

12.5V

G

S

125pF

125pF

100 kHz

OSCILLATOR

OFF

ON

GATE CLAMP

ZENER

Figure 10. Gate Control

Circuit Detail

Q1

Q4

Q6

Q5

Q3

Q2