SC4502HEVB datasheet(9/19 Pages) SEMTECH

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 2005 Semtech Corp.

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POWER MANAGEMENT

SC4502/SC4502H

The absolute maximum operating frequency of the

converter is therefore

1.67MHz

150ns

0.25

150ns

MIN

. The

actual operating frequency needs to be lower to allow

for modulating headroom.

The power transistor inside the SC4502/SC4502H is

turned off every switching cycle for an interval determined

by the discharge time of the oscillator ramp plus the

propagation delay of the power switch. This minimum off

time limits the maximum duty cycle of the regulator at a

given switching frequency. A boost converter with high

OUT

ratio requires long switch on time and high duty cycle.

If the required duty cycle is higher than the attainable

maximum, the converter will operate in dropout. (Dropout

is the condition in which the regulator cannot attain its

set output voltage below current limit.)

The minimum off times of closed-loop boost converters set

to various output voltages were measured by lowering their

input voltages until dropout occurs. It was found that the

minimum off time of the SC4502/SC4502H ranged from

80ns to 110ns at room temperature.

Beware of dropout while operating at very low input

voltages (1.5V-2V) with off time approaching 110ns.

Shorten the PCB trace between the power source and

the device input pin, as line drop may be a significant

percentage of the input voltage. A regulator in dropout

may appear as if it is in current limit. The cycle-by-cycle

current limit of the SC4502/SC4502H is duty-cycle and

input voltage invariant and is typically 2A. If the switch

current limit is not at least 1.4A, then the converter is

likely in dropout. The switching frequency should then be

lowered to improve controllability.

Both the minimum on time and the minimum off time

reduce control range of the PWM regulator. Bench

measurement showed that reduced modulating range

started to be a problem at frequencies over 2MHz. Although

the oscillator is capable of running well above 2MHz,

controllability limits the maximum operating frequency.

Inductor Selection

The inductor ripple current

∆I

L of a boost converter

operating in continuous-conduction mode is

()

CESAT

⋅

−

⋅

∆

(5)

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

Substituting (3) into (5) and neglecting V

CESAT ,

(6)

In peak 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 DI

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

good compromise. Inductors so chosen are optimized in

size and DCR. Setting

∆I

L = 0.3•(1.4A) = 0.42A, VD=0.5V

in (6),









−

⋅









−

∆

⋅

0.5V

0.42A

OUT

(7)

where L is in

µH and f is in MHz.

Equation (6) shows that for a given V

OUT, ∆IL is the highest

when

()

OUT

. 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 (2A). 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.

Input Capacitor

The input current in a boost converter is the inductor

current, which is continuous with low RMS current ripples.

A 2.2µF-4.7µF ceramic input capacitor is adequate for

most applications.

Output Capacitor

Both ceramic and low ESR tantalum capacitors can be

used as output filtering capacitors. Multi-layer ceramic

capacitors, due to their extremely low ESR (<5m

Ω), are

the best choice. Use ceramic capacitors with stable

Applications Information









−

⋅

∆

OUT

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