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 2004 Semtech Corp.

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POWER MANAGEMENT

SC2618

Theory of Operation

The SC2618 is a hysteretic mode PWM controller. It uses

a comparator to generate PWM wave with minimum on

time and off time control. As shown in the block diagram,

the output voltage is fed back to the comparator nega-

tive input and compared to a setting voltage. If the out-

put voltage is below its set point, the top gate drive will

turn on and remain on until the minimum on time has

expired AND the output voltage has risen above the set

point. Similarly, if the output voltage is above its set point,

top gate drive will be turned off and bottom gate will turn

on and stay on until the minimum off time has expired

AND the output is below its set point. Because of this

minimum time control scheme, the comparator hyster-

esis is not required and is internally set to zero.

Switching F

Switching F

Switching F

Switching F

Switching Freq

req

req

req

requency vs. Duty Cy

uency vs. Duty Cy

uency vs. Duty Cy

uency vs. Duty Cy

uency vs. Duty Cycle

cle

cle

cle

cle

This control scheme will force a buck converter to oper-

ate either at minimum on time, or minimum off time, or

both. The SC2618 has minimum time of 1us. Its switch-

ing frequency, peaking at 500kHz, can be found in Fig. 1

as long as the voltage ratio of the buck converter has

been decided.

0

0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.8 0.9 1.0

100

200

300

400

500

Duty Cycle

To

n=

1u

s

Tof

f=1

us

Fig. 1. Switching frequency vs. duty cycle

UVL

UVL

UVL

UVL

UVLO, Star

O, Star

O, Star

O, Star

O, Start up and Shut do

t up and Shut do

t up and Shut do

t up and Shut do

t up and Shut down

wn

wn

wn

wn

To initiate the SC2618, supply voltages are applied to

Vcc and BST pins. The top gate (DH) and bottom gate

(DL) are held low until Vcc exceed UVLO threshold, typi-

cally 4.5V. At that point, the internal soft start capacitor

begins to charge, the top gate remains low, and the bot-

tom gate is pulled high to turn on the bottom FET. When

the voltage at soft start cap reaches a setting level, the

top and bottom gates of PWM controller begin to switch.

The switching regulator output is slowly ramping up for a

soft turn on. If, however, one or more supply voltage rails

are late or absent, the output voltage will not rise and

the part will behave as if there was a short circuit at the

output, that is, it will go into hiccup mode. It will remain in

this mode until all the necessary voltage rails are present,

at which time normal operation will start.

If the supply voltage Vcc falls below UVLO threshold dur-

ing normal operation, the soft start capacitor begins to

discharge. When the voltage reaches the setting level,

the PWM controller control the switching regulator out-

put to ramp down slowly for a soft turn off.

Hiccup Mode Shor

Hiccup Mode Shor

Hiccup Mode Shor

Hiccup Mode Shor

Hiccup Mode Short Cir

t Cir

t Cir

t Cir

t Circuit Pr

cuit Pr

cuit Pr

cuit Pr

cuit Proooootttttection

ection

ection

ection

ection

Short circuit protection is implemented by comparing the

feedback node with the soft start and reference volt-

age. If the FB voltage falls more than about 200mV be-

low it’s correct voltage the short circuit latch is set which

immediately disables the top drive. If the short circuit

occurred during soft start, the soft start capacitor con-

tinues to charge to it’s full voltage before discharging

towards zero. If the short occurred during steady state

operation, the internal soft start capacitor begins to dis-

charge immediately, at the same rate as it charged, to-

wards zero. Once the soft start capacitor reaches its

minimum voltage, the latches are cleared and a normal

soft start is attempted, if the short circuit condition has

not been cleared, the fault latch will again be set once

the soft start voltage reaches about 200mV and the cycle

will repeat until the short is cleared.

It is also possible, with large output capacitor values,

that the output voltage will be unable to rise fast enough

to accurately follow the soft start voltage and a short

circuit trip may result. During soft start the bottom gate

drive is disabled to prevent discharging the output ca-

pacitor and so the output capacitor will retain the volt-

age achieved before the trip and will charge to a higher

voltage during the next hiccup cycle. In this way the out-

put voltage will achieve the set point value but may take

several hiccup cycles to do so.