SC4503 datasheet(10/22 Pages) SEMTECH

10 / 22 page

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POWER MANAGEMENT

SC4503

−

(5)

Assuming that the voltage of a nearly discharged Li-ion cell

is 2.6V. Using V

D=0.5V, VCESAT=0.3V and D=0.86 in (5),

−

•

−

Transient headroom requirement further reduces the maxi-

mum achievable output voltage to below 16V.

Minimum Controllable On-Time

The operating duty cycle of a boost converter decreases as

IN approaches VOUT. Sensed switch current ramp modulates

the pulse width in a current-mode switching regulator. This

current ramp is absent unless the switch is turned on. The

intersection of this ramp with the error ampliﬁer output

determines the switch on-time. The propagation delay

time required to immediately turn off the switch after it is

turned on is the minimum controllable on time. Measured

minimum on time of the SC4503 is load-dependent and

ranges from 180ns to 220ns at room temperature. The

switch in the SC4503 is either not turned on, or, for at least

this minimum. If the regulator requires a switch on-time

less than this controllable minimum, then it will either skip

cycles or start to jitter.

Inductor Selection

The inductor ripple current ΔI

L of a boost converter in con-

tinuous-conduction mode is

(

)

−

∆

(6)

where f is the switching frequency and L is the induc-

tance.

Substituting (3) into (6) and neglecting V

CESAT,

−

∆

(7)

In current-mode control, the slope of the modulating

(sensed switch current) ramp should be steep enough to

Applications Information (Cont.)

lessen jittery tendency but not so steep that large ﬂux swing

decreases efﬁciency. For continuous-conduction mode

operation, inductor ripple current ΔI

L between 0.35A and

0.6A is a good compromise. Setting ΔI

L = 0.43A, VD = 0.5V

and f = 1.3MHz in (7),

−

∆

(8)

where L is in μH.

Equation (7) shows that for a given V

OUT, ΔIL is the highest

when

(

)

. If V

IN varies over a wide range, then

choose L based on the nominal input voltage.

The saturation current of the inductor should be 20-30%

higher than the peak current limit (1.9 A). Low-cost powder

iron cores are not suitable for high-frequency switching

power supplies due to their high core losses. Inductors

with ferrite cores should be used.

Discontinuous-Conduction Mode

The output-to-input voltage conversion ratio

continuous-conduction mode is limited by the maximum

duty cycle D

MAX:

−

Higher voltage conversion ratios can be achieved by oper-

ating the boost converter in full-time discontinuous-con-

duction mode (DCM). Deﬁne

OUT

as the equivalent

output load resistance. The following inequalities must be

satisﬁed for DCM operation:

−

(9)

and,

(10)

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