SC197 datasheet(12/20 Pages) SEMTECH

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SC197

General Description

The SC197 contains two identical synchronous step-down

PWM (Pulse Width Modulated) DC-DC regulators. Each

regulator utilizes a 3.5MHz fixed-frequency voltage mode

architecture. Each is designed to operate in fixed-fre-

quency PWM mode and enter PSAVE (Power Save) mode

utilizing pulse frequency modulation under light load

conditions to maximize efficiency. Each regulator requires

only two capacitors and a single inductor to be imple-

mented in most systems. The switching frequency has

been chosen to minimize the size of the inductor and

capacitors while maintaining high efficiency. Output

voltage is programmable, eliminating the need for exter-

nal programming resistors. Loop compensation is also

internal, eliminating the need for external components to

control stability.

Programmable Output Voltage

The SC197 has 15 fixed output voltage levels which can be

individually selected by programming the CTLx(A/B)

control pins (see Table 1 on page 2 for settings). Control

pins with an “A” suffix refer to the A output, and the “B”

suffix refers to the B output. “A”and“B”devices are electri-

cally isolated and share no connections internal to the

package. The“A”or“B”device is disabled whenever all four

CTLxA or all four CTLxB pins are pulled low. The “A” or “B”

device is enabled whenever at least one of the CTLxA or

CTLxB pins is pulled high. This configuration eliminates

the need for a dedicated enable pin. Each CTLx(A/B) pin is

internally pulled down via 1MΩ if V

IN is below 1.5V or if the

voltage on the control pin is below the input high voltage.

This ensures that the output is disabled when power is

applied if there are no inputs to the CTLx(A/B) pins. Each

weak pull-down is disabled whenever its pin is pulled high

and remains disabled until all CTLx(A/B) pins are pulled

low.

The output voltage can be set using different methods. If

a static output voltage is required, the CTLx(A/B) pins can

be tied to either IN or GND to set the desired voltage

whenever power is applied at IN. If enable control is

required, each CTLx(A/B) pin can be tied to either GND or

to a microprocessor I/O line to create the desired control

code whenever the control signal is forced high. This

approach is equivalent to using the CTLx(A/B) pins collec-

tively as a single enable pin. A third option is to connect

each of the four CTLx(A/B) pins to individual micropro-

cessor I/O lines. Any of the 15 output voltages can be

programmed using this approach. If only two output

voltages are needed, the CTLx(A/B) pins can be combined

in a way that will reduce the number of I/O lines to 1, 2,

or 3, depending on the control code for each desired

voltage. Other CTLx(A/B) pins could be hard wired to

GND or IN. This option allows dynamic voltage adjust-

ment for systems that reduce the supply voltage when

entering sleep states. Note that applying all zeros to the

CTLx(A/B) pins when changing the output voltages will

temporarily disable the device, so it is important to avoid

this combination when dynamically changing levels.

CTLxA and CTLxB pins may only be connected together

when A and B devices share the same power source; i.e.,

INA and INB pins are connected together, and GNDA and

GNDB are connected together. Note that connecting any

CTLxA and CTLxB pins together will force both A and B

d e v i ce s to m a k e o u t p u t vo l t a g e c h a n g e s

simultaneously.

Adjustable Output Voltage Selection

If an output voltage other than one of the 15 program-

mable settings is needed, an external resistor divider

network can be added to the SC197 to adjust the output

voltage setting. This network scales the output based on

the resistor ratio and the programmed output setting.

The resistor values can be determined using the equa-

tion. Note that V

OUT may refer to either the A or B device.

LEAK

SET

OUT

where V

OUT is the desired output voltage, VSET is the

voltage setting selected by the CTLx(A/B) pins, R

FB1 is the

resistor between the output capacitor and the OUT(A/B)

pin, R

FB2 is the resistor between the OUT(A/B) pin and

ground, and I

LEAK is the leakage current into the OUT(A/B)

pin during normal operation. The current into the

OUT(A/B) pin is typically 1µA, so the last term of the equa-

tion can be neglected if the current through R

FB2 is much

larger than 1µA. Selecting a resistor value of 10kΩ or

Applications Information

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