MIC2155 datasheet(16/35 Pages) MICREL

16 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

CBST

Figure 9. Gate Drive

dv/dt Induced Turn On of the Low-Side MOSFET

As the high-side MOSFET turns on, the rising dv/dt on

the switch-node forces current through CGD of the low-

side FET causing a glitch on the FET’s gate. Figure 10

illustrates the basic mechanism causing this issue. If the

glitch on the gate is greater than the FET’s turn-on

threshold, it may cause an unwanted turn-on of the low-

side FET while the high-side FET is on. A short circuit

between input and ground would occur that lowers

efficiency and increases power dissipation in both FETs.

Additionally, turning on the low-side FET during the off-

time could interfere with overcurrent sensing.

VDS

Figure 10. dv/dt Induced Turn-On

The following steps can be taken to lower the gate drive

impedance, minimize the dv/dt induced current and

lower the FETs susceptibility to the induced glitch:

1) Chose a MOSFET with:

a) a high CGS/CGD ratio

b) a low internal gate resistance

2) Do not put a resistor between the LSD output

and the gate.

3) Insure both the gate drive and return etch are

short, low inductance connections.

4) Use a 4.5V VGS-rated MOSFET because its

higher gate threshold voltage is more immune to

glitches than a 2.5V or 3.3V rated FET.

5) Connect VDD to VIN or a 5V supply if VIN is below

6V. The RDSON of the internal driver will be lower

and a 4.5V rated MOSFET can be used.

Remote Sense

Remote sensing provides accurate output voltage

regulation by sensing at the load. Remote sensing

makes up for losses in the power distribution path. It

uses a unity gain differential amplifier to overcome

voltage drops in both the output and return (ground)

paths. The amplifier has common mode input range from

-0.3V to 3.6V. For proper remote sense operation, VIN

must be greater than 6V. If VIN is less than 6V, the VDD

pin must be connected to VIN or externally supplied with

5V.

The output of the remote sense amplifier can source up

to 500µA. The voltage divider resistors (R1 and R4 in

Figure 12) must be chosen to insure the output current

of the amplifier does not exceed the maximum of 500µA.

500

REF

OUT

MAX

−

≥

Where:

VREF = 0.7V

A gain/phase plot of the remote sense amplifier in Figure

11 shows a typical 2MHz bandwidth. Phase lag is 45° at

1MHz.

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