MIC2185

Micrel, Inc.

MIC2185

8

October 2005

MOSFET, Q2. Current flows through the MOSFET because

its voltage drop is less than diode D1. The MOSFET remains

on until the end of the switching cycle. There is another

non-overlap time delay between the turn-off of the high side

MOSFET and the turn-on of the low side MOSFET at the

beginning of the next switching cycle.

The description of the MIC2185 controller is broken down

into 6 basic functions.

• Control Loop

• PWM Operation

• SKIP Mode Operation

• Current Limit

• MOSFET gate drive

• Reference, Enable & UVLO

• Oscillator & Sync

• Soft Start

Control Loop

PWM and SKIP modes of operation

The MIC2185 can operate in either PWM (pulse width modu-

lated) or skip mode. The efficiency of the boost converter can

be improved at lower output loads by manually selecting the

skip mode of operation. The potential disadvantage of skip

mode is the variable switching frequency that accompanies

this mode of operation. The occurrence of switching pulses

depends on component values as well as line and load con-

ditions. PWM mode is the best choice of operation at higher

output loads. In skip mode, current through the inductor can

settle to zero, causing voltage ringing across the inductor.

PWM mode has the advantages of lower output ripple volt-

age and higher efficiencies at higher output loads. Another

advantage of the synchronous PWM mode of operation is

that the inductor current is always continuous, even at zero

output current. This reduces parasitic ringing that occurs

Osc

Q2

D1

Q1

L1

Control

Bias

Skip Current

Limit Comparator

Hysteresis

Comparator

1%

Current Reset

On

PGND

12

CSH

9

OUTN

13

OUTP

14

V

16

1

FB

5

SGND

6

VDD

10

VREF

8

COMP

4

SS

3

SYNC

11

FREQ/2

15

SKIP

2

EN/UVLO

7

50mV

1.245V

R2

R1

Figure 2. MIC2185 Skip Mode Block Diagram