MIC2168AYMM datasheet(8/18 Pages) MICREL

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Micrel, Inc.

MIC2168A

Functional Description

The MIC2168A is a voltage mode, synchronous step-

down switching regulator controller designed for high

output power 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 1MHz switching regulator.

Theory of Operation

The MIC2168A is a voltage mode step-down regulator.

The block diagram illustrates the voltage control loop.

The output voltage variation due to load or line changes

will

sensed

the

inverting

input

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 0 to 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, let us assume the output

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

cause the inverting input of the error amplifier which is

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.

Soft-Start

The COMP pin on the MIC2168A is used for the

following three functions:

Disables the part by grounding this pin

External compensation to stabilize the voltage

control loop

Soft-start

For better understanding of the soft-start feature, let’s

assume VIN = 12V, and the MIC2168A is allowed to

power-

up. The COMP pin has an internal 8.5μA current

source

that

charges

the

external

compensation

capacitor. As soon as this voltage rises to 180mV (t =

Cap_COMP ×

0.18V/8.5μA), the MIC2168A allows the

internal VDD linear regulator to power up and as soon as

it crosses the undervoltage lockout of 2.6V, the chip’s

internal oscillator starts switching. At this point in time,

the COMP pin current source increases to 40μA and an

internal 11-bit counter starts counting which takes

approximately 2ms to complete. During counting, the

COMP voltage is clamped at 0.65V. After this counting

cycle the COMP current source is reduced to 8.5µA and

the COMP pin voltage rises from 0.65V to 0.95V, the

bottom edge of the saw-tooth oscillator. This is the

beginning of 0% duty cycle and it increases slowly

causing the output voltage to rise slowly. The MIC2168A

has two hysteretic comparators that are enabled when

VOUT is within ±3% of steady state. When the output

voltage reaches 97% of programmed output voltage,

then the gm error amplifier is enabled along with the

hysteretic comparator. This point onwards, the voltage

control loop (gm error amplifier) is fully in control and will

regulate the output voltage.

Soft-start time can be calculated approximately by

adding the following four time frames:

t1 = Cap_COMP ×

0.18V/8.5μA

t2 = 12 bit counter, approx 2ms

t3 = Cap_COMP ×

0.3V/8.5μA

8.5

μ.

Cap_COMP

0.5

OUT













Soft-Start Time(Cap_COMP = 100nF) = t1 + t2 + t3 + t4

= 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms

Current Limit

The MIC2168A uses the RDS(ON) of the top power

MOSFET to measure output current. Since it uses the

drain to source resistance of the power MOSFET, it is

not very accurate. This scheme is adequate to protect

the power supply and external components during a

fault condition by cutting back the time the top MOSFET

is on if the feedback voltage is greater than 0.67V. In

case of a hard short when feedback voltage is less than

0.67V, the MIC2168A discharges the COMP capacitor

to 0.65V, resets the digital counter and automatically

shuts off the top gate drive, and the gm error amplifier

and the –3% hysteretic comparators are completely

disabled and the soft-start cycles restarts. This mode of

operation is called the “hiccup mode” and its purpose is

to protect the downstream load in case of a hard short.

The circuit in Figure 1 illustrates the MIC2168A current

limiting circuit.

April 22, 2015

Revision 3.0

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