MIC2159 datasheet(6/17 Pages) MICREL

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Micrel

MIC2159

October 2006

M9999-101206

Block Diagram

MIC2159 Block Diagram

Functional Description

The MIC2159 is a voltage mode, synchronous step-

down switching regulator controller designed for high

power. Current limit is implemented without the use of an

external sense resistor. It includes an internal soft-start

function which reduces the power supply input surge

current at start-up by controlling the output voltage rise

time, a PWM generator, a reference voltage, two

MOSFET drivers, and short-circuit current limiting

circuitry to form a complete 400kHz switching regulator.

Theory of Operation

The MIC2159 is a voltage mode step-down regulator.

The figure above illustrates the block diagram for the

voltage control loop. The output voltage variation due to

load or line changes will be sensed by the inverting input

of the transconductance error amplifier via the feedback

resistors R3, and R2 and compared to a reference

voltage at the non-inverting input. This will cause a

small change in the DC voltage level at the output of the

error amplifier which is the input to the PWM

comparator. The other input to the comparator is a 1v

triangular waveform. The comparator generates a

rectangular waveform whose width tON is equal to the

time from the start of the clock cycle t0 until t1, the time

the triangle crosses the output waveform of the error

amplifier. To illustrate the control loop, assume the

output voltage drops due to sudden load turn-on, this

would cause the inverting input of the error amplifier,

which is the divided down version of VOUT, to be slightly

less than the reference voltage, causing the output

voltage of the error amplifier to go high. This will cause

the PWM comparator to increase tON time of the top side

MOSFET, causing the output voltage to go up and

bringing VOUT back in regulation. If this sudden load

transient was large enough to cause a 3% change in

output voltage, then the output of the Hysteretic

comparator will bypass the PWM comparator and drive

the LSD and HSD outputs at full Duty cycle in order to

recover the nominal output voltage in the fastest manner

possible whilst fixed frequency PWM switching is

maintained during normal loading.

Soft-Start

The COMP/EN pin on the MIC2159 is used for the

following three functions:

1. Disables the part by grounding this pin.

2. External compensation to stabilize the voltage

control loop.

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