April 2005

9

M9999-042205

MIC2186

Micrel, Inc.

SKIP Mode Operation

This control method is used to improve efficiency at low

output loads. A block diagram of the MIC2186 SKIP mode is

shown in Figure 2. The power drawn by the MIC2186 control

IC is (I

the IC can be a significant portion of the total output power

during periods of low output current, which lowers the effi-

ciency of the power supply. In SKIP mode the MIC2186

lowers the IC supply current by turning off portions of the

control and drive circuitry when the IC is not switching. The

disadvantage of this method is greater output ripple and

variable switching frequency. The soft start, HiDC and Sync

pins have no effect when operating in SKIP mode.

In SKIP mode, switching starts when the feedback voltage

drops below the lower threshold level of the hysteresis

comparator. The OutN pin goes high, turning on the N-

channel MOSFET, Q1. Current ramps up in the inductor until

either the current limit comparator or the hysteretic voltage

comparator turns off Q1’s gate drive. If the feedback voltage

exceeds the upper hysteretic threshold, Q1’s gate drive is

terminated. However, if the voltage at the CSH pin exceeds

the SKIP mode current limit threshold, it terminates the gate

drive for that switching cycle. The gate drive remains off for

a constant period at the end of each switching cycle. This off

time period is typically 1µs when the F/2 pin is low and 2µs

when the F/2 pin is high. Figure 3 shows some typical SKIP

mode switching waveforms.

I_inductor

T

ON

T

PER

V

COMP

I_inductor

I_inductor

Gate Driver

I_inductor

Voltage

Divider

Gate Drive at OutN

Figure 3. SKIP Mode Waveform

The SKIP mode current threshold limits the peak inductor

current per cycle. Depending on the input, output and circuit

parameters, many switching cycles can occur before the

feedback voltage exceeds the upper hysteretic threshold.

Once the voltage on the feedback pin exceeds the upper

hysteretic threshold the gate drive is disabled. The output

load discharges the output capacitance causing Vout to

decrease until the feedback voltage drops below the lower

threshold voltage limit. The switching converter then turns the

gate drive back on. While the gate drive is disabled, the

MIC2186 draws less IC supply current then while it is switch-

ing, thereby improving efficiency at low output loads. Figure

4 shows the efficiency improvement at low output loads when

SKIP mode is selected.

0

10

20

30

40

50

60

70

80

00.05

0.1

0.15

0.2

0.25

INPUT CURRENT (A)

Low Current

Efficiency

SKIP Mode

PWM Mode

Figure 4.

The maximum peak inductor current depends on the skip

current limit threshold and the value of the current sense

resistor, Rsense. For a typical 50mV current limit threshold in

SKIP Mode, the peak inductor current is:

I

50mV

R

INDUCTOR_pk

SENSE

=

The maximum output current is SKIP mode depends on the

input conditions, output conditions and circuit component

values. Assuming a discontinuous mode where the inductor

current starts from zero at each cycle, the maximum output

current is calculated below:

I

2.5 10

L

fs

2R

V

V

O(max)

3

SENSE

2

O

IN

=

×× ×

××

− ×

()

−

η

where:

Iomax is the maximum output current

Vo is the output voltage

Vin is the input voltage

L is the value of the boost inductor

fs is the switching frequency

η is the efficiency of the boost converter

Rsense it the value of the current sense resistor

PWM Operation

Figure 5 shows typical waveforms for PWM mode of opera-

tion. The gate drive signal turns on the external MOSFET

which allows the inductor current to ramp up. When the

MOSFET turns off, the inductor forces the MOSFET drain

voltage to rise until the boost diode turns on and the voltage

is clamped at approximately the output voltage.