SC453TSTRT datasheet(20/26 Pages) SEMTECH

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POWER MANAGEMENT

SC453

Applications Information (Cont.)

To calculate the maximum DC value of current the maxi-

mum DC current must be added to the maximum ripple

value to obtain peak current:

It is recommended that the current limit be set at 120%

of the peak value to allow for inductor current overshoot

during load transients.

The Current Limit Comparator internal to the SC453 moni-

tors the output current and turns the high side switch off

when the current exceeds the upper current limit thresh-

old, I

CLMAX

and re-enables only if the load current drops be-

low the lower current limit threshold, I

CLMIN

. The current is

sensed by monitoring the voltage drop across the current

sense resistor R

Current limiting will cycle from I

CLMAX

to I

CLMIN

for 32 switch-

ing cycles to allow for short term transients, then the con-

verter is latched off. I

CLMAX

and I

CLMIN

are set according to

the following equations.

The current limit is set at I

CLIM

= ICLMAX, and then solve

for R

, which is R6 in the typical applications circuit. For

balance, R8, in series with the CLRF pin is kept the same

value as R6.

RCL

ICLIM RHYS

⋅

RCS

⋅

2.5 VREF

⋅

RCL

673.59

681

Step 7 — Small Capacitors/Resistor Selection

Several small capacitors are required for signal filtering.

Refer to the Typical Application Circuit on page 2. Use

SMT ceramic capacitors with an X7R or better tempera-

ture coefficient. COG is preferred.

C11 and R7 are used to filter the hysteretic ripple signal.

The components are sized to provide filtering above the

5th harmonic of the fundamental.

C11

⋅ R7

⋅

C11

9.095

−

In the evaluation board design a 100pF ceramic capacitor

is used for C11. The DAC output requires a 1nF capacitor

(C23) for high frequency noise filtering. The values for

C12 and C13 are calculated in a similar manner, though

they are returned to the CORE pin because that is the ref-

erence point for the current limit comparator.

C12

⋅ R6

⋅

C13

C12

1.335

−

C13

1.335

−

IPEAK

23.389 A

IPEAK

IMAX_FL

IRIPPLE_MAX

ICLIM

120 % IPEAK

⋅

ICLIM

28.066 A

ICLMAX

3VREF

⋅

RCL

RHYS RCS

⋅

ICLMIN

2VREF

⋅

RCL

RHYS RCS

⋅

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