SC284 datasheet(12/20 Pages) SEMTECH

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SC284

Applications Information

Detailed Description

The SC284 is a two channel synchronous step-down

converter. Both channels on this device are designed to

operate in fixed-frequency PWM mode at 2.5MHz and

provide the same current capacity of up to .8A. The

switching frequency is chosen to minimize the size of the

external inductor and capacitors while maintaining high

efficiency. Both channels of SC284 are independent.

Operation

During normal operation, the PMOS MOSFET is activated

on each rising edge of the internal oscillator. The voltage

feedback loop uses an internal feedback resistor divider.

The period is set by the internal oscillator. The device

has an internal synchronous NMOS rectifier and does

not require a Schottky diode on the LX pin. The device

operates as a buck converter in PWM mode with a fixed

frequency of 2.5MHz.

Programmable Output Voltage

Both channels on SC284 have fifteen pre-determined

output voltage values which can be individually selected

by programming the CTL input pins (see Table — Output

Voltage Settings). Each CTL pin has an active MΩ internal

pull-down resistor. The MΩ resistor is switched in circuit

whenever the CTL input voltage is below the input

threshold, or when the part is in under-voltage lockout. It is

recommended to tie all high CTL pins together and use an

external pull-up resistor toVIN if there is no enable signal, or

if the enable input is an open drain/collector signal.The CTL

pins may be driven by a microprocessor to allow dynamic

voltage adjustment for systems that reduce the supply

voltage when entering sleep states. Avoid all zeros being

present on the CTL pins when changing programmable

output voltages as this would disable the device.

SC284 is also capable of regulating a different (higher)

output voltage, which is not shown in the Table , via an

external resistor divider. There will be a typical 2µA cur-

rent flowing into the VOUT pin. The typical schematic for

an adjustable output voltage option from the standard

.0V with CTLx=[000], is shown in Figure . RFBA/B

and RFB2A/B are used to adjust the desired output volt-

age. If the RFB2A/B current is such that the 2µA VOUT

pin current can be ignored, then RFBA/B can be found

by the next equation. RFB2A/B needs to be low enough

in value for the current through the resistor chain to be

at least 20µA in order to ignore the VOUT pin current.

OSTD

OUT

where V

OSTD is the pre-determined output voltage via the

CTL pins.

FF is needed to maintain good transient response

performance. The correct value of C

FF can be found using

the following Equation.

)

(

]

[

]

[

OSTD

OUT

To simplify the design, it is recommended to program the

desired output voltage from a standard .0V as shown in

Figure with the correct C

FF calculated from Equation 2.

For programming the output voltage from other standard

voltages, R

FB, RFB2 and CFF need to be adjusted to meet

Equations and 2.

SC284

CTL0B

CTL1B

CTL2B

CTL3B

Enable B

CTL0A

CTL1A

CTL2A

CTL3A

Enable A

PGNDA

PGNDB

RFB1B = (VOUTB-1)

x RFB2B

for CTLBX = 0010

(1.0V)

AVINB

VINB

CAVINB

0.1µF

RAVINB1Ω

PVINB

CINB

10µF

AGNDB

AVINA

VINA

CAVINA

0.1µF

RAVINA1Ω

PVINA

CINA

10µF

AGNDA

VOUTB

COUTB

LXB

VOUTB

RFB2B

10kΩ

CFFB

RFB1B

VOUTA

COUTA

LXA

VOUTA

RFB2A

10kΩ

CFFA

RFB1A

RFB1A = (VOUTA-1)

x RFB2A

for CTLAX = 0010

(1.0V)

Figure 1 — Output Voltage Programming

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