April 2005

7

M9999-062205

MIC2168A

Micrel, Inc.

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

within ±3% of steady state. When the output voltage reaches

is enabled along with the hysteretic comparator. This point

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

V

V

OUT

IN

t4









x 0.5

-

x

Cap_COMP

8.5µA

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

t4 = 2.1ms + 2ms + 3.5ms + 1.8ms = 10ms

Current Limit

to measure output current. Since it uses the drain to source

DS(ON)

DS(ON)

resistance of the power MOSFET, it is not very accurate. This

MOSFET

MOSFET

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

–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 down stream

load in case of a hard short. The circuit in Figure 1 illustrates

the MIC2168A current limiting circuit.

L1 Inductor

HSD

LSD

RCS

CS

200µA

0

C2

C1

Q1

MOSFET N

Q2

MOSFET N

Figure 1. The MIC2168A Current Limiting Circuit

ing equation:

R

CS

=

200µA

R

Equation (1)

I

L

=

2(Inductor Ripple Current)

1

I

LOAD

=

where:

Inductor Ripple Current =

(V

IN

V

OUT

=

- V

OUT

- V

- V

)

V

IN

F

SWITCHING

L

×

×

200µA is the internal sink current to program the

MIC2168A current limit.

therefore, it is recommended to add a 50% margin to the load

DS(ON)

DS(ON)

limiting due to increased MOSFET junction temperature rise.

DS(ON)

DS(ON)

filter some of the switching noise.

is generated through a low-dropout regulator and generates

supply to the input supply through a 10Ω resistor for input

supplies between 2.9V to 5V.

MOSFET Gate Drive

The MIC2168Ahigh-side drive circuit is designed to switch an

N-Channel MOSFET. The block diagram in Figure 2 shows a

bootstrap circuit, consisting of D2 and CBST, supplies energy

to the high-side drive circuit. Capacitor CBSTis charged while

the low-side MOSFET is on and the voltage on the VSW pin

is approximately 0V. When the high-side MOSFET driver is

turned on, energy from CBST is used to turn the MOSFET

on. As the MOSFET turns on, the voltage on the VSW pin

and CBST floats high while continuing to keep the high-side

MOSFET on. When the low-side switch is turned back on,

CBST is recharged through D2. The drive voltage is derived

gate drive voltage is 5V and the nominal high-side gate drive

voltageisapproximately4.5VduethevoltagedropacrossD2.

An approximate 20ns delay between the high- and low-side

driver transitions is used to prevent current from simultane-

ously flowing unimpeded through both MOSFETs.

MOSFET Selection

The MIC2168A controller works from input voltages of 3V to

13.2V and has an internal 5V regulator to provide power to

turn the external N-Channel power MOSFETs for high- and