MIC2185 Datasheet, PDF (8/15 Pages) Micrel Semiconductor – Low Voltage Synchronous Boost PWM Control IC

MIC2185
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
V_IN
C_DECOUP
V_INA ₁
EN/UVLO
7
V_DD
SKIP
2
Bias
On
Control
Micrel
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
conditions. PWM mode is the best choice of operation at
higher output loads. In skip mode, current through the induc-
tor can settle to zero, causing voltage ringing across the
inductor. PWM mode has the advantages of lower output
ripple voltage and higher efficiencies at higher output loads.
Another advantage of the synchronous PWM mode of opera-
tion is that the inductor current is always continuous, even at
C_IN
L1
V_REF
V_DD
V_INP
16
P_GND
OUTP
14
D1
Q2
V_OUT
C_OUT
FREQ/2
15
SYNC
11
SS
3
Osc
OUTN
Q1
13
50mV
P_GND
Current Reset
Skip Current
Limit Comparator
CSH
9
R_SENSE
COMP
4
P_GND
V_REF
1.245V
PGND
12
VREF
8
V_REF
R1
FB
6
Hysteresis
R2
VDD
10
V_DD
Comparator
±1%
S_GND
SGND ⁵
MIC2185
Figure 2. MIC2185 Skip Mode Block Diagram
8
May 2002