Micrel, Inc.

MIC45116 Evaluation Board

SW Node

A test pad (SW) is placed close to the module for

monitoring the switching waveform. It is one of the most

critical waveforms for the converter.

Current Limit

and external resistor connected from the ILIM pin to the

SW node to decide the current limit.

VIN

SW

ILIM

PGND

C5

R26

C16

PVIN

MIC45116

Figure 1. MIC45116 Current-Limiting Circuit

In each switching cycle of the MIC45116, the inductor

current is sensed by monitoring the low-side MOSFET in

with the power ground (PGND) after a blanking time of

150ns. In this way the drop voltage over the R26 resistor

generating the short current limit. The small capacitor

(C16) connected from ILIM pin to PGND filters the

switching node ringing during the off-time allowing a better

short-limit measurement. The time constant created by

R26 and C16 should be much less than the minimum off

time.

value of the resistor (R26) if the absolute value of the

event is triggered. A hiccup cycle to treat the short event is

generated. The hiccup sequence including the soft-start

reduces the stress on the switching FETs and protects the

load and supply for severe short conditions.

The short-circuit current limit can be programmed by using

Equation 3:

( )

CL

CL

DS(ON)

PP

L

CLIM

I

V

R

0.5)

ΔI

(I

R26

+

×

×

−

=

Eq. 3

where:

side power MOSFET, 16mΩ

typically

14mV per Electrical Characteristics in the MIC45116 data

sheet)

per Electrical Characteristics in the MIC45116 data sheet).

ΔI

is integrated, use Equation 4 to calculate the inductor

ripple current.

The peak-to-peak inductor current ripple is:

L

f

V

)

V

(V

V

I

sw

IN(MAX)

OUT

IN(MAX)

OUT

L(PP)

×

×

−

×

=

∆

Eq. 4

The MIC45116 has 1.0µH inductor integrated into the

module. In case of hard short, the short limit is folded

down to allow an indefinite hard short on the output without

any destructive effect. It is mandatory to make sure that

the inductor current used to charge the output capacitance

during soft start is under the folded short limit; otherwise

the supply will go in hiccup mode and may not be finishing

the soft start successfully.

in the above equation to avoid false current limiting due to

increased MOSFET junction temperature rise. With R26 =

1.62k

Ω and C16 = 15pF, the typical output current limit is

about 8.2A.

March 12, 2015

3

Revision 1.0