October 2005

9

MIC2185

MIC2185

Micrel, Inc.

during the discontinuous mode of operation found in lightly

loaded, non-synchronous boost converters. Pulling the SKIP

pin (pin 2) low will force the controller to operate in PWM

mode for all load conditions. Pulling the SKIP pin high will

force the controller to operate in SKIP mode.

Skip Mode Operation

This control method is used to improve efficiency at low output

loads. A block diagram of the MIC2185 skip mode is shown

in Figure 2. The power drawn by the MIC2185 control IC is

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

periods of low output current, which lowers the efficiency of

the power supply. In skip mode the MIC2185 lowers the IC

supply current by disabling the high side drive and running

at lower than the PWM switching frequency. It also turns 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 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 com-

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

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

the SKIP mode 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. Or, 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 1us when the F/2 pin is low and

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

switching waveforms in SKIP mode.

SKIP Mode Waveform

TIME (50µs/div)

Switch NodeVoltage

(Low Side FET Drain)

5V/div

Low Side FET

Gate Drive

5V/div

V

200mV/div

Inductor Current

5A/div

V

V

Figure 3. SKIP mode waveforms

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

MIC2185 draws less IC supply current then while it is switch-

ing, thereby improving efficiency at low output loads.

Figure 4 shows the improvement in efficiency that SKIPmode

makes when at lower output currents.

0

20

40

60

80

100

OUTPUT CURRENT (A)

MIC2185 PWM vs.

Skip Mode Efficiency

P W M

400kHz

S K I P

Figure 4.

The maximum peak inductor current depends on the skip

current limit threshold and the value of the current sense

in skip mode, the peak inductor current is:

I

50mV

R

INDUCTOR_pk

SENSE

=

The maximum output current in 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

2 R

V

V

O(max)

3

SENSE

2

O

IN

=

×

× ×

×

×

− ×

(

)

(

)

−

η

L is the value of the boost inductor

η

is the efficiency of the boost converter