MIC2185_05 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, Inc.
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
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
13
Q1
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
Figure 2. MIC2185 Skip Mode Block Diagram
MIC2185
8
October 2005