MIC2155 datasheet(16/33 Pages) MICREL

16 / 33 page

Micrel, Inc.

MIC2155/2156

May 2009

M9999-052709-A

(408) 944-0800

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.

FREQUENCY (Hz)

-10

-20

-30

-40

200

160

120

-40

-80

-120

-160

-200

10k

100k

10M

Phase

Magnitude

Figure 11. Remote Sense Amplifier Gain/Phase Plot

A typical remote sense configuration is shown in Figure

12. The output of the remote sense amplifier feeds a

voltage divider (R1, R4), which is connected to the

Channel 1 error amplifier. The divider and compensation

network for the remote sense are the same as for a local

sense configuration. The 10Ω resistors provide an

alternate feedback path if the remote sense connections

are removed or opened. The remote sense connections

should not be shorted or the output voltage will increase

close to VIN. The OVP circuit in the controller will not

protect against this type of fault since the feedback pin

voltage will be 0V.

LOAD

+SENSE

–SENSE

Figure 12. Remote Sense

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