SC4530 datasheet(8/23 Pages) SEMTECH

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SC4530

General Description and Operation

The SC4530 is a micropower, hysteretic current-mode

step-down switching regulator. As shown in the block

diagram in Figure 2, the converter is controlled by an error

amplifier EA and two current-sensing comparators IPK and

IVLY. IPK and IVLY monitor the switch (Q

) collector current

and the freewheeling diode (D

) current respectively. The

EA amplifies the differential voltage between the FB and

the bandgap reference, and produces a current, I

CNTL,

proportional to its output voltage. I

CNTL, in turn, adjusts the

switching thresholds of both the peak and valley current

comparators. The EA output voltage is high at heavy

loads, as is the peak inductor current. The Zener diode

Z clamps the amplifier output and sets the switch peak

current limit.

When the switch Q

is turned on, the current through Q

ramps up until it reaches the peak threshold set by I

CNTL. The

output of the IPK comparator, OC, goes high. This resets

the latch and turns off the switch. With Q

off, the inductor

current ramps down through the freewheeling diode

. When D current ramps below the valley threshold

established by I

CNTL, the output of the IVLY comparator,

UC, goes high. If Q

has been turned off for more than

530ns, then the latch will be set and Q

will again turn on,

starting a new cycle.

The inductor ripple current in continuous-conduction

mode is independent of I

CNTL and is primarily determined

by V

OS and VHYS. Continuous mode switching frequency,

therefore, depends on V

IN, VOUT, the inductance L and the

propagation delay times of the current comparators.

If the regulator output is shorted to ground, then the

amplifier output will rise to D

Z clamp voltage. Q turns

off as the inductor current reaches the peak current limit.

With the output shorted to ground, the inductor current

ramps down at a slower rate through D

. Q turns on again

when the inductor current crosses the valley threshold.

Therefore, short-circuiting the output merely lowers the

converter switching frequency. The inductor current

remains bounded by the peak switch current limit.

The RUN/IDLE comparator, CMP, monitors the output of

the error amplifier. If the EA output falls below the RUN/

IDLE threshold, then Q

and all control circuits except the

reference and EA will be shut off. The output capacitor

will then supply the load, causing the output voltage

to fall. When the EA output rises above the RUN/IDLE

threshold, the control circuit wakes up and the part starts

to switch, delivering power to the output. The offset

voltage V

OS at the input of the IPK comparator ensures

that any current pulse delivered to the output has some

minimum amplitude.

At very light loads, even a single minimum charge packet

delivered to the output will cause the FB voltage to rise

above the reference voltage. This causes the EA output

voltage to fall and the part to idle. The part resumes

switching when the output current discharges the FB

voltage below the reference. At light loads, the part

switches only as needed to keep the output in regulation.

By reducing the supply current drawn when idling, high

efficiency is maintained at light loads. At heavier loads,

it may take a number of consecutive minimum pulses to

bring the FB above the reference voltage. The part enters

continuous conduction mode when the amplifier output

never falls below the RUN/IDLE threshold.

Driving the base of the power transistor above the input

power supply rail minimizes the power transistor turn-

on voltage and maximizes efficiency. A bootstrap circuit

[formed by an internal bootstrap diode D

2 (Figure 2) and

an external capacitor connected between BST and SW]

generates a voltage higher than V

IN at the BST pin. The

bootstrapped voltage becomes the supply voltage of the

power transistor driver.

The internal control circuit takes its power from either the

input or from the BIAS pin if V

BIAS > 2.3V. For applications

with output voltage higher than 2.5V, the BIAS pin should

be tied to the regulator output to maximize efficiency.

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