SC121 datasheet(17/23 Pages) SEMTECH

17 / 23 page

SC121

Applications Information (continued)

PWM Operation

The PWM cycle runs at a fixed frequency (f

osc

= 1.2MHz),

with a variable duty cycle (D). PWM operation continually

draws current from the input supply, except for low

output loads in which current flows periodically from, and

back into, the input. During the on-state of the PWM

cycle, the n-channel FET is turned on, grounding the

inductor at the LX pin. This causes the current flowing

from the input supply through the inductor to ground to

ramp up. During the off-state, the n-channel FET is turned

off and the p-channel FET (synchronous rectifier) is turned

on. This causes the inductor current to flow from the

input supply through the inductor into the output capaci-

tor and load, boosting the output voltage above the input

voltage. The cycle then repeats to re-energize the

inductor.

Ideally, the steady state (constant load) duty cycle is

determined by D = 1 – (V

OUT

), but must be greater in

practice to overcome dissipative losses. The SC121 PWM

controller constrains the value of D such that 0.20 < D < 0.90

(approximately).

The average inductor current during the off-state multi-

plied by (1-D) is equal to the average load current. The

inductor current is alternately ramping up (on-state) and

down (off-state) at a rate and amplitude determined by

the inductance value, the input voltage, and the on-time

= D×T, T = 1/f

OSC

). Therefore, the instantaneous induc-

tor current will be alternately larger and smaller than the

average.

If the average output current is sufficiently small, the

minimum inductor current can ramp down to zero during

the off-state. Discontinuous mode operation (where both

FETs turn off as the inductor current reaches zero) is not

supported in the SC121, since this would result in a finite

positive minimum current from input to output, which

would cause an uncontrolled rise in output voltage in this

case. Instead, the inductor current will reverse for the

remainder of the off-state, flowing from the output

capacitor into the OUT pin, through the p-channel FET to

the LX pin, and through the inductor to the input capaci-

tor. Negative inductor current ripple allows regulation

even with zero output load. The energy returned to the

input capacitor is not wasted, but dissipative conduction

losses will inevitably occur.

The minimum on-time limitation imposes a minimum

boost ratio, so if V

is too close to V

OUT

> V

OUT

– 400mV,

approximately), V

OUT

will rise above the programmed

value for a sufficiently small output load. A higher output

load requires a higher duty cycle to overcome dissipative

losses, such that regulation at programmed V

OUT

will

eventually be restored. But this regulation-restoration

load rises rapidly with V

, so this phenomenon can be

beneficially exploited in only rare circumstances. If opera-

tion with high V

and low load is required, please consider

using the SC120, a pin compatible dual mode (PWM/

PSAVE) boost converter. The SC120 will support zero load

in PSAVE mode for V

up to V

OUT

+ 150mV.

Regulator Startup, Short Circuit Protection,

and Current Limits

The SC121 permits power up at input voltages from 0.85V

to 4.5V. Soft-start startup current limiting of the internal

switching n-channel and p-channel FET power devices

protects them from damage in the event of a short

between OUT and GND. As the output voltage rises, pro-

gressively less-restrictive current limits are applied. This

protection unavoidably prevents startup into an exces-

sive load.

Upon enable, the p-channel FET between the LX and OUT

pins turns on with its current limited to approximately

150mA, the short-circuit output current. When V

OUT

approaches V

(but is still below 1.7V), the n-channel

current limit is set to 350mA (the p-channel limit is dis-

abled), the internal oscillator turns on (approximately

200kHz), and a fixed 75% duty cycle PWM operation

begins. (See the section PWM Operation.) When the

output voltage exceeds 1.7V, fixed frequency PWM opera-

tion begins, with the duty cycle determined by an n-

channel FET peak current limit of 350mA. When this

n-channel FET startup current limit is exceeded, the on-

state ends immediately and the off-state begins. This

determines the duty cycle on a cycle-by-cycle basis.

When V

OUT

is within 2% of the programmed regulation

voltage, the n-channel FET current limit is raised to 1.2A,

and normal voltage regulation PWM control begins.

Once normal voltage regulation PWM control is initiated,

the output becomes independent of V

and output regu-

lation can be maintained for V

as low as 0.7V, subject to

the maximum duty cycle and peak current limits. The

SC121 Datasheet(PDF) 17 Page - Semtech Corporation

SC121 Datasheet(HTML) 17 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL