Micrel, Inc.

MIC28512 Evaluation Board

April 8, 2015

3

Revision 1.0

A “hiccup” soft-start cycle is generated, reducing the stress

on the power switching FETs while protecting the load and

supply during 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

)

5

.

0

ΔI

(I

22

R

+

×

×

=

-

Eq. 3

Where:

28mΩ typically

14mV, per the Electrical Characteristics section in the

MIC28512 datasheet)

per the Electrical Characteristics section in the MIC28512

datasheet).

ΔI

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

is reduced by half to the short-circuit threshold. This allows

an indefinite hard short on the output without any

destructive effect. It is critical to make sure that the

inductor current used to charge the output capacitance

during soft start is below the foldback short-circuit level;

otherwise the supply can go into hiccup mode and latch up

at start up. This should be verified over the operating

temperature range as well.

in Equation 4 to avoid false current limiting due to

increased MOSFET junction temperature rise. Table 1

shows typical output current limit value for a given R22.

Table 1. R22 Typical Output Current-Limit Value

R22

Typical Output Current Limit

2.21

kΩ

4.3A

1.82

kΩ

3.0A

The MIC28512 evaluation board was designed with a

8.2µH inductor for operation at 300kHz at 5V output. The

44

mΩ.

Setting the Switching Frequency

The MIC28512 switching frequency can be adjusted by

changing the value of resistor R17. The top resistor (R19)

is set at 100k

Ω and is connected between VIN and FREQ.

R4 is connected from the FREQ input to PGND and sets

the switching frequency according to Equation 4.

Figure 2. Switching Frequency Adjustment

17

R

19

R

17

R

f

f

O

ADJ

_

SW

+

×

=

Eq. 4

Where:

Electrical

Characteristics

section

in

the

MIC28512

datasheet.

For a more precise setting, it is recommended to use the

Figure 3:

Figure 3. Switching Frequency vs. R17