SC283 datasheet(12/20 Pages) SEMTECH

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SC283

Applications Information

Detailed Description

The SC283 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 1.8A. The

switching frequency is chosen to minimize the size of the

external inductor and capacitors while maintaining high

efficiency. Both channels of SC283 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 SC283 have fifteen pre-determined

output voltage values which can be individually selected

by programming the CTL input pins (seeTable 1 — Output

Voltage Settings). Each CTL pin has an active 1 MΩ

internal pulldown resistor. The 1MΩ resistor is switched in

circuit whenever the CTL input voltage is below the input

threshold, or when the part is in undervoltage lockout.

It is recommended to tie all high CTL pins together and

use an external pull-up resistor to VIN 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.

SC283 is also capable of regulating a different (higher)

output voltage, which is not shown in the Table 1, via an

external resistor divider.There will be a typical 2µA current

flowing into theVOUT pin.The typical schematic for an ad-

justableoutputvoltageoptionfromthestandard1.0Vwith

CTLx=[0010], is shown in Figure 1. RFB1 and RFB2 are used

to adjust the desired output voltage. If the RFB2 current is

such that the 2µA VOUT pin current can be ignored, then

RFB1 can be found by Equation 1. RFB2 needs to be low

enough in value for the current through the resistor chain

to be at least 20µA in order to ignore theVOUT pin current.

OSTD

OUT

⋅

−

(1)

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

Equation 2.

(

)

(

)

(

]

[

]

[

−

⋅

Ω

−

OSTD

OUT

(2)

To simplify the design, it is recommended to program the

desired output voltage from a standard 1.0V as shown in

Figure 1 with the correct C

FF calculated from Equation 2.

For programming the output voltage from other standard

voltages, R

FB1, RFB2 and CFF need to be adjusted to meet

Equations 1 and 2.

CTL0_

CTL1_

CIN_

10µF

VIN

LX_

VOUT_

Enable

SC283

(Channel A or B)

VIN

CTL2_

GND

CTL3_

VOUT_

COUT_

RFB1 CFF

RFB2

)

(

OUT

−

for CTLx= 0010 (1.0V)

Figure 1 — Typical Schematic for Adjusting the

Output Voltage Up from an Output Voltage of 1.0V

(CTLx=[0010])

Maximum Power Dissipation

Each channel of SC283 has its own Θ

JA of 65°C/W when

only one channel is in operation. Since both channels are

withinsamepackage,thereisabout50%heatwhichwillbe

transferred to the adjacent channel. The equivalent total

thermal impedence will be higher when the neighboring

channel is also in operation. To guarantee an operating

junction temperature of less than 125°C, Figure 2 shows

the maximum allowable power loss of each channel. The

curve is based upon the junction temperature of either

channel reaching a maximum of 125°C. Each channel of

SC283 can support up to 1.8A load current. Figures 3a

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