SC202A datasheet(12/19 Pages) SEMTECH

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SC202A

General Description

The SC202A is a synchronous step-down PWM (Pulse

Width Modulated) DC-DC regulator utilizing a 3.5MHz

fixed-frequency voltage-mode architecture and an inter-

nal 1µH inductor. The device is designed to operate in

fixed-frequency PWM mode and enter PSAVE (power save)

mode utilizing pulse frequency modulation under light

load conditions for maximizing efficiency. Two capacitors

are the only external components required — one for

input decoupling and one for output filtering. The 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 SC202A has 15 fixed output voltage levels which can

be individually selected by programming the CTL control

pins (CTL3-0 — see Table 1 on page 2 for settings). The

device is disabled whenever all four CTL pins are pulled

low and enabled whenever at least one of the CTL pins is

pulled high. This configuration eliminates the need for a

dedicated enable pin. Each CTL 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 CTL pins. Each weak pull-down is dis-

abled whenever its pin is pulled high and remains disabled

until all CTL pins are pulled low.

The output voltage can be set using different methods. If

a static output voltage is required, the CTL pins can be

tied to either IN or GND to set the desired voltage when-

ever power is applied at IN. If enable control is required,

each CTL pin can be tied to either GND or to a micropro-

cessor I/O line to create the desired control code whenever

the control signal is forced high. This method is equivalent

to using the CTL pins collectively as a single enable pin. A

third option is to connect each of the four CTL pins to

individual microprocessor I/O lines. Any of the 15 output

voltages can be programmed using this method. If only

two output voltages are needed, the CTL pins can be com-

bined 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.

Dynamic Output Voltage Adjustment

Dynamically changing the CTL pins allows dynamic

voltage adjustment for systems that reduce the supply

voltage when entering sleep states. This should done

using specific procedures. Attention needs to be made so

that applying all zeros in a very short period to the CTL

pins when changing the output voltage will temporarily

disable the device. Therefore it is important to avoid

these combinations of all zero when dynamically chang-

ing the CTL levels. For example, when the CTLs change

from 0001 to 0010 (0.8V to 1.2V), a transitional state of

0000 (shut down) state might occur in a very short period

of time, which could result the device to be disabled

unintentionally. In order to achieve such operation, the

correct logic transition stages should be 001000110001

(1V1.2V0.8V). The 0011 (1.2V) state should be kept short

to prevent from the 0000 state.

If the output voltage level is not within the specified the

CTL voltage levels, the resistor divider ratio can be

switched by a logic voltage level through an external

MOSFET to achieve the dynamic output voltage

transition.

Secondly, when the CTL pin is toggled for the output

voltage to increase, the regulator will increase the induc-

tor current and force the output voltage follow. When

the CTL pin is toggled for the output voltage to decrease,

the regulator will force the output voltage go down

immediately (at heavy load conditions) when the device

is in CCM. If the device is in DCM operation (the inductor

current does not go negative) then the output voltage

will go down as the load current drains the output

capacitor.

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 SC202A to adjust the output

voltage setting. This network scales the output based on

the resistor ratio and the programmed output setting.

Applications Information

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