5

Oct10-07 RevJ

SP6652 1A, High Efficiency, Current Mode PWM Buck Regulator

© 2007 Sipex Corporation

Current Mode Control and Slope

Compensation

The SP6652 is designed to use low value

ceramic capacitors and low value inductors

to reduce the converter’s volume and cost

in portable devices. Current mode PWM

control was, therefore, chosen for the ease

of compensation when using ceramic output

capacitors and better transient line rejec-

tion, which is important in battery powered

applications. Current mode control spreads

the two poles of the output power train filter

far apart so that the modulator gain crosses

over at -20dB/decade instead of the usual

-40dB/decade. The external compensation

network is, simply, a series RC circuit con-

nected between ground and the output of the

internal transconductance error amplifier.

It is well known that an unconditional insta-

bility exists for any fixed frequency current-

mode converter operating above 50% duty

cycle. A simple, constant-slope compensa-

tionischosentoachievestabilityunderthese

conditions. The most common high duty

cycle application is a Li-Ion battery powered

regulator with a 3.3V output (D ≥ 90%). Since

thecurrentloopiscriticallydampedwhenthe

the amount of slope compensation chosen

is, therefore:

-702mA/µs

The inductor current is sensed as a voltage

across the PMOS charging switch and the

NMOS synchronous rectifier (see BLOCK

DIAGRAM). Duringinductorcurrentcharge,

rent ramp times the resistance of the PMOS

chargingswitch.Tokeeptheeffectivecurrent

slopecompensationconstant(remembering

current is being compensated, not voltage)

the voltage slope must be proportional to

pensation voltage is internally generated

with a bias current that is also proportional

Over Current Protection

In steady state closed loop operation the

voltage at the COMP pin controls the duty

cycle.Duetothecurrentmodecontrolandthe

slope compensation, this voltage will be:

The COMP node will be clamped when its

at the output of the translator. The correct

value of clamp voltage is, therefore:

keep the value of current limit approximately

constant over process and temperature

by a peak-holding circuit that senses the

amplitude of the slope compensation ramp

even if the COMP node is at zeroV, since

the peak current comparator is reset at the

end of a charge cycle and is held low during

a blanking time after the start of the next

charge cycle. This is necessary to swamp

the transients in the inductor current ramp

(100ns, nominally) is not sufficient for the

COMP node to keep control of the current

DetaileD Description