SC187

Applications Information

Detailed Description

The SC187 is a synchronous step-down PWM (Pulse Width

Modulated) DC-DC converter utilizing a 2.2MHz fixed-fre-

quency voltage mode architecture. The device is designed

to operate in fixed-frequency PWM mode. The switching

frequency is chosen to minimize the size of the external

inductor and capacitors while maintaining high

efficiency.

Operation

During normal operation, the PMOS MOSFET is activated

on each rising edge of the internal oscillator. The period is

set by the onboard 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

2.2MHz.

Protection Features

The SC187 provides the following protection features:

Current Limit

Over-Voltage Protection

Soft-Start Operation

Thermal Shutdown

Current Limit & OCP

The internal PMOS power device in the switching stage is

protected by a current limit feature. If the inductor current

is above the PMOS current limit for 16 consecutive cycles,

the part enters foldback current limit mode and the output

current is limited to the current limit holding current

(I

the output voltage will be the product of I

load resistance. The SC187 is capable of sustaining an

indefinite short circuit without damage. During the soft

start, if current limit has occurred before the SS voltage

has reached 400mV, the part enters foldback current limit

mode. Foldback current limit mode will be disabled during

soft-start after the SS voltage is higher than 400mV.

Over-Voltage Protection

In the event of a 15% over-voltage on the output, the

PWM drive is disabled with the LX pin floating. Switching

•

•

•

•

does not resume until the output voltage falls below the

nominal V

Programmable Output Voltage

The SC187 has fifteen pre-determined output voltage

values which can be individually selected by program-

ming the CTL input pins (see Table 1 — Output Voltage

Settings). Each CTL pin has an active 500kΩ internal pull-

down resistor. The 500kΩ resistor is switched in circuit

whenever the CTL input voltage is below the input thresh-

old, 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 to AVIN 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 program-

mable output voltages as this would disable the device.

SC187 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 the VOUT pin. The typical schematic for an

adjustable output voltage option from the standard 1.0V

with CTLx=[0010], is shown in Figure 2. 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 using the next equation.

RFB2 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.

CTL0

CTL1

PGND

AVIN

LX

VOUT

SC187

PVIN

CTL2

AGND

CTL3

PGOOD

SSA

10nF

L

10kΩ

(1.0V)

10nF

22µF

100kΩ

Enable

Figure 1 — Output Voltage Programming

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