MIC2155YML datasheet(18/35 Pages) MICREL

18 / 35 page

Micrel, Inc.

MIC2155/2156

November 2009

M9999-111209-B

The MIC2155/6 current limit circuit restricts the

maximum output current. If the load tries to draw

additional current the output voltage drops until it is no

longer within regulation limits. At this point (75% of

nominal output voltage) a hiccup current mode is

initiated to protect down stream loads from excessive

current during hard short circuits. This helps reduce the

overall power dissipation in the PWM converter

components during a fault.

Figure 14. Overcurrent Sense Waveforms

The MIC2155/6 only senses current across the low side

MOSFET of Channel 1 since both channels operate in

parallel. This means the total output current limit is

approximately twice the calculated current limit.

Current Limit Setting

The current limit circuit responds to the peak inductor

current flowing through the low-side FET. The value of

RCS can be estimated with the “simple” method or can be

more accurately calculated by taking the inductor ripple

current into account.

The Simple Method

Current limit can be quickly estimated with the following

equation:

RCS = IOUT/2 × RDSON(MAX)/180µA.

Where: RDSON is the maximum on-resistance of the low

side FET at the operating junction temperature

Accurate Method

For designs where ripple current is significant when

compared to IOUT or for low duty cycle operation,

calculating the current setting resistor RCS should take

into account that we are sensing the peak inductor

current and that there is a blanking delay of

approximately 100ns.

Figure 15. Overcurrent waveform

The equations to accurately calculate the current limit

resistor value are shown below:

RIPPLE

OUT

)

(

OUT

RIPPLE

−

DLY

OUT

SET

−

)

MIN

(

)

MAX

(

DSON

SET

D = Duty Cycle

FS = Switching Frequency

L = Power inductor value

TDLY = Current limit blanking time ~ 100ns

ICS(min) = 180µA

Example:

Consider a 12V to 3.3V @ 30A converter with 1.5µH

power inductor and 90% efficiency at full load. Each

channel will supply 15A at a 500kHz (MIC2155)

switching frequency. The on-resistance of the low side

MOSFET is 6mΩ.

Using the simple method:

500

180

RCS

MIC2155YML Datasheet(PDF) 18 Page - Micrel Semiconductor

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