SC2620 datasheet(10/26 Pages) SEMTECH

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 2006 Semtech Corp.

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SC2620

POWER MANAGEMENT

that the minimum on time is about 105ns at room

temperature (Figure 5). The power switch in the SC2620

is either not turned on at all or for at least T

ON(MIN). If the

required switch on time (=

) is shorter than the minimum

on time, the regulator will either skip cycles or it will jitter.

Example: Determine the maximum operating frequency

of a dual 24V to 1.2V and 24V to 3.3V switching regulator

using the SC2620.

Assuming that V

D = 0.45V, VCESAT = 0.25V and VIN = 26.4V

(10% high line), the corresponding duty ratios, D

1 and D2,

of the 1.2V and 3.3V converters can be calculated using

(2).

062

−

To allow for transient headroom, the minimum operating

switch on time should be at least 30% higher than the

worst-case minimum on time exhibited in Figure 5.

Designing for a switch on time of 150ns at

IN =

the maximum operating frequency of the 24V to 1.2V and

3.3V converter is

kHz

410

150

Minimum Off Time Limitation

The PWM latch in Figure 2 is reset every period by the

clock. The clock also turns off the power transistor to refresh

the bootstrap capacitor. This minimum off time limits the

attainable duty cycle of the regulator at a given switching

frequency. The measured minimum off time is 120ns. For

a step-down converter, D increases with increasing

OUT

ratio. If the required duty cycle is higher than the attainable

maximum, then the output voltage will not be able to reach

its set value in continuous-conduction mode.

Applications Information

Example: Determine the maximum operating frequency of

a dual 5V to 1.5V and 5V to 4V switching regulator using

the SC2620.

Assuming that V

D = 0.45V, VCESAT = 0.25V and VIN = 4.5V

(10% low line), the duty ratios

D and

D of the 1.5V and

4V converters can be calculated using (2).

−

The maximum operating channel frequency of the dual

1.5V and the 4V converter is therefore

kHz

410

120

−

Transient headroom requires that channel frequency be

lower than 410kHz.

Inductor Selection

The inductor ripple current

∆I

L for a non-synchronous step-

down converter in continuous-conduction mode is

)

(

)

)(

(

)

)(

(

CESAT

OUT

−

∆

(3)

where f is the switching frequency and L is the inductance.

In current-mode control, the slope of the modulating

(sensed switch current) ramp should be steep enough to

lessen jittery tendency but not so steep that large flux

swing decreases efficiency. Inductor ripple current

∆I

between 25-40% of the peak inductor current limit is a

good compromise. Inductors so chosen are optimized in

size

and

DCR.

Setting

)

(

∆

D =

and

CESAT =

in (3),

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