SC4501 datasheet(8/21 Pages) SEMTECH

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POWER MANAGEMENT

SC4501

Operation

The SC4501 is a programmable constant-frequency peak

current-mode step-up switching regulator with an

integrated 2A power transistor. Referring to the block

diagrams in Figures 2 and 3, the power transistor is

switched on at the trailing edge of the clock. Switch

current is sensed with an integrated sense resistor. The

sensed current is summed with the slope-compensating

ramp before compared to the output of the error

amplifier EA. The PWM comparator trip point determines

the switch turn-on pulse width. The current-limit

comparator ILIM turns off the power switch when the

switch current exceeds the 2.8A current-limit threshold.

ILIM therefore provides cycle-by-cycle current limit.

Current-limit is not affected by slope compensation

because the current comparator ILIM is not in the PWM

signal path.

Current-mode switching regulators utilize a dual-loop

feedback control system. In the SC4501 the amplifier

output COMP controls the peak inductor current. This is

the inner current loop. The double reactive poles of the

output LC filter are reduced to a single real pole by the

inner current loop, easing loop compensation. Fast

transient response can be obtained with a simple Type-2

compensation network. In the outer loop, the error

amplifier regulates the output voltage.

The switching frequency of the SC4501 can be programmed

up to 2MHz with an external resistor from the ROSC pin

to the ground. For converters requiring extreme duty

cycles, the operating frequency can be lowered to

maintain the necessary minimum on time or the minimum

off time.

The SC4501 requires a minimum input of 1.4V to operate.

A voltage higher than 1.1V at the shutdown pin enables

the internal linear regulator REG in the SC4501. After V

REG

becomes valid, the soft-start capacitor is charged with a

1.5

µA current source. A PNP transistor clamps the output

of the error amplifier as the soft-start capacitor voltage

rises. Since the COMP voltage controls the peak inductor

current, the inductor current is ramped gradually during

soft-start, preventing high input start-up current. Under

fault conditions (V

IN<1.4V or over temperature) or when

the shutdown pin is pulled below 1.1V, the soft-start

capacitor is discharged to ground. Pulling the shutdown

pin below 0.1V reduces the total supply current to 10

µA.

Setting the Output Voltage

An external resistive divider R

1 and R2 with its center tap

tied to the FB pin (Figure 4) sets the output voltage.













−

242

OUT

(1)

VOUT

40nA

SC4501

The input bias current of the error amplifier will introduce

an error of:

(

)

242

100

OUT

∆

(2)

The percentage error of a V

OUT = 5V converter with R1 =

100K

Ω and R

2 = 301KΩ is

(

)

242

100

301

100

OUT

∆

Operating Frequency and Efficiency

Switching frequency of SC4501 is set with an external

resistor from the ROSC pin to the ground. A graph showing

the relationship between R

OSC and switching frequency is

given in the “Typical Characteristics”.

High frequency operation reduces the size of passive

components but switching losses are higher. The efficiencies

of 5V to 12V converters operating at 700KHz, 1.35MHz

and 2MHz are shown in Figure 1(b). The peak efficiency

of the SC4501 appears to decrease 0.5% for every

100KHz increase in frequency.

Application Information

Figure 4. The Output Voltage is set with a Resistive Divider

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