MIC5012
Micrel
Applications Information
(Continued)
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
+
Q3
Q1
125pF
125pF
Q5
C1
Q2
100 kHz
OSCILLATOR
OFF
ON
Q4
C2
G
500Ω
GATE CLAMP
ZENER
12.5V
Q6
S
Figure 10. Gate Control
Circuit Detail
5-122
April 1998
MICREL [ MICREL SEMICONDUCTOR ]
