November 2003

7

M9999-111803

MIC2168

Micrel

0.65V. After this counting cycle the COMP current source is

reduced to 8.5

µA 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 MIC2168 has

two hysteretic comparators that are enabled when V

within

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

97% of programmed output voltage, then the g

fier is enabled along with the hysteretic comparator. This

point onwards, the voltage control loop (g

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

t4

V

V

0.5

Cap_COMP

8.5 A

OUT

IN

=









××

µ

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

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

Current Limit

The MIC2168 uses the R

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 MIC2168 discharges the COMP capacitor to

0.65V, resets the digital counter and automatically shuts off

the top gate drive, and the g

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 MIC2168 current limiting circuit.

L1 Inductor

HSD

LSD

RCS

CS

200µA

0

C2

C1

Q1

MOSFET N

Q2

MOSFET N

Figure 1. The MIC2168 Current Limiting Circuit

The current limiting resistor R

equation:

R

RI

200 A

CS

DS(ON) Q1

L

=

×

µ

Equation (1)

II

1

2 Inductor Ripple Current

L

LOAD

where:

Inductor Ripple Current = V

V

– V

VF

L

OUT

IN

OUT

IN

SWITCHING

×

()

××

F

200

µA is the internal sink current to program the MIC2168

current limit.

The MOSFET R

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

current (I

limiting due to increased MOSFET junction temperature rise.

It is also recommended to connect R

drain of the top MOSFET Q1, and the R

source of Q1 to accurately sense the MOSFETs R

0.1

µF capacitor in parallel with R

filter some of the switching noise.

Internal V

The MIC2168 controller internally generates V

biasing and to provide power to the gate drives. This V

DD

supply is generated through a low-dropout regulator and

generates 5V from V

voltage less than 5V, the V

mately 200mV in dropout. Therefore, it is recommended to

short the V

resistor for input supplies between 2.9V to 5V.

MOSFET Gate Drive

The MIC2168 high-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 CBST is

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 increases to approximately V

reversed biased 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 from the internal 5V V

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

high-side gate drive voltage is approximately 4.5V due the

voltage drop across D2. An approximate 20ns delay between

the high- and low-side driver transitions is used to prevent

current from simultaneously flowing unimpeded through both

MOSFETs.

MOSFET Selection

The MIC2168 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