SC186 datasheet(13/18 Pages) SEMTECH

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SC186

Applications Information

Detailed Description

The SC186 is a synchronous step-down PWM (Pulse Width

Modulated) DC-DC converter utilizing a 1.5MHz fixed-fre-

quency voltage mode architecture. The device is designed

to operate in fixed-frequency PWM mode and will enter

PSAVE (power save) mode at light loads to maximize effi-

ciency. The switching frequency is chosen to minimize the

size of the external inductor and capacitors while main-

taining 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 when in PWM mode. 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 1.5MHz at medium to high loads. At

light loads the part will enter PSAVE mode to maximize

efficiency.

Power Save Mode Operation

When the load current decreases below the PSAVE thresh-

old, PWM switching stops and the device automatically

enters PSAVE mode. This threshold varies depending upon

the input voltage and output voltage setting, optimizing

efficiency for all possible load currents. While in PSAVE

mode, output voltage regulation is controlled by a series of

bursts in switching. During a burst, the inductor current is

limited to a peak value which controls the on-time of the

PMOS switch. After reaching this peak, the PMOS switch is

disabled and the inductor current is forced to near 0mA.

Switching bursts continue until the output voltage climbs

to V

OUT +2% or until the PSAVE current limit is reached.

Switching is then stopped to eliminate switching losses,

enhancing overall efficiency. Switching resumes when the

output voltage reaches the lower threshold of V

OUT and

continues until the upper threshold again is reached. Note

that the output voltage is regulated hysteretically while in

PSAVE mode between V

OUT and VOUT + 2%. The period and

duty cycle while in PSAVE mode are solely determined by

IN and VOUT until PWM mode resumes. This can result in the

switching frequency being much lower than the PWM

mode frequency.

If the output load current increases enough to cause V

OUT

to decrease below the PSAVE exit threshold (V

OUT - 4%),

the device automatically exits PSAVE and operates in con-

tinuous PWM mode. Note that the PSAVE high and low

threshold levels are both set at or above V

OUT to minimize

undershoot when the SC186 exits PSAVE. Figure 1 illus-

trates the transitions from PWM mode to PSAVE mode and

back to PWM mode.

VOUT

Load

Demand

(

IOUT)

BURST

OFF

VLX

Time

PWM Mode at

Load

PSAVE Mode at

Light Load

PWM Mode at

Medium / High

Load

PSAVE

EXIT

VOUT +2%

VOUT -4%

Medium / High

Figure 1 — Transitions between PWM and PSAVE Modes

Protection Features

The SC186 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

CL_HOLD) which is approximately 1A. Under this condition,

the output voltage will be the product of I

CL_HOLD and the

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