KB3426B-1.8 datasheet(6/16 Pages) KINGBOR

6 / 16 page

–

IRCMP

–

ICOMP

RUN

OSC

SLOPE

COMP

OSC

FREQ

SHIFT

0.6V

0.6V REF

SHUTDOWN

0.4V

0.65V

SLEEP

VIN

VFB/VOUT

BURST

VIN

RS LATCH

SWITCHING

LOGIC

AND

BLANKING

CIRCUIT

ANTI-

SHOOT-

THRU

GND

(Refer to Functional Diagram)

Main Control Loop

The KB3426 uses a constant frequency, current mode

step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel MOSFET) switches

are internal. During normal operation, the internal top

power MOSFET is turned on each cycle when the oscillator

sets the RS latch, and turned off when the current com-

parator, ICOMP, resets the RS latch. The peak inductor

current at which ICOMP resets the RS latch, is controlled by

the output of error amplifier EA. When the load current

increases, it causes a slight decrease in the feedback

voltage, FB, relative to the 0.6V reference, which in turn,

causes the EA amplifier’s output voltage to increase until

the average inductor current matches the new load cur-

rent. While the top MOSFET is off, the bottom MOSFET is

turned on until either the inductor current starts to reverse,

as indicated by the current reversal comparator IRCMP, or

the beginning of the next clock cycle.

Burst Mode Operation

The KB3426 is capable of Burst Mode operation in which

the internal power MOSFETs operate intermittently based

on load demand.

In Burst Mode operation, the peak current of the inductor

is set to approximately 200mA regardless of the output

load. Each burst event can last from a few cycles at light

loads to almost continuously cycling with short sleep

intervals at moderate loads. In between these burst events,

the power MOSFETs and any unneeded circuitry are turned

off, reducing the quiescent current to 20

µA. In this sleep

state, the load current is being supplied solely from the

output capacitor. As the output voltage droops, the EA

amplifier’s output rises above the sleep threshold signal-

ing the BURST comparator to trip and turn the top MOSFET

on. This process repeats at a rate that is dependent on the

load demand.

KB3426

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TEL:(86)0755-26508846 FAX:(86)0755-26509052

SIMPLIFIED BLOC DIAGRAM

OPERATION

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