MIC2185_05 Datasheet, PDF (7/15 Pages) Micrel Semiconductor – Low Voltage Synchronous Boost PWM Control IC

MIC2185
Functional Diagram
V_IN
C_DECOUP
1 ^VIN^A
EN/UVLO
7
Bias
SKIP
2
fs/4
FREQ/2
15
SYNC
Osc
11
Control
Correction
Ramp
SS
3
On
Overcurrent Reset
PWM
Comparator
C_IN
V_REF
V_DD
V_INP
16
P_GND
OUTP
14
0.1V
P_GND
OUTN
13
Overcurrent
Comparator
CSH
9
COMP
4
Error
Amplifier
gm = 0.0002
Gain = 20
100k
Gain = 3.7
V_REF
P_GND
0.3V
PGND
12
L1
D1
Q2
C_OUT
Q1
R_SENSE
Micrel, Inc.
V_OUT
fs/4
VREF
8
V_REF
R1
FB
6
Frequency
R2
VDD
10
V_DD
Foldback
S_GND
SGND ⁵
Figure 1. MIC2185 PWM Mode Block Diagram
Functional Description
The MIC2185 is a BiCMOS, switched mode, synchronous
boost (step up) control IC. The synchronous switched, high
side P-channel MOSFET, Q2, placed in parallel with the out-
put diode, D1, improves the efficiency of the boost converter.
The lower voltage drop across the MOSFET reduces power
dissipation and increases efficiency. Current mode control is
used to achieve superior transient line and load regulation.
An internal corrective ramp provides slope compensation
for stable operation above a 50% duty cycle. The controller
is optimized for high efficiency, high performance DC-DC
converter applications.
Figure 1 is a block diagram of the MIC2185 configured as
a PWM synchronous boost converter. The switching cycle
starts when OutN goes high and turns on the low side, N-
channel MOSFET, Q1. The Vgs of the MOSFET is equal to
VinP. This forces current to ramp up in the inductor. The
inductor current flows through the current sense resistor,
Rsense. The voltage across the resistor is amplified and
combined with an internal ramp for stability. This signal is
compared with the comp output signal of the error amplifier.
When the current signal equals the error voltage signal, the
low side MOSFET is turned off. The inductor current then
flows through the diode, D1, to the output. A delay between
the turn-off of the low side MOSFET and the turn-on of the
high side MOSFET prevents both MOSFETs from being on
at the same time, which would short the output to ground.
At the end of the non-overlap time, OutP pulls the gate of
the MOSFET to ground, turning on the high side, P-channel
October 2005
7
MIC2185