Datasheet

LTC3114-1
13
Rev. C
For more information www.analog.com
OPERATION
Current Mode Control
The LTC3114-1 utilizes average current mode control for
the pulse width modulator as shown in Figure2. Current
mode control, both average and the better known peak
method, enjoy some benefits compared to other control
methods including: simplified loop compensation, rapid
response to load transients and improved rejection of line
voltage transients.
Referring to Figure2, an internal high gain transconduc
-
tance error amplifier monitors V
OUT
through a voltage
divider connected to the FB pin and provides an output, VC,
that is used by the current mode control loop to command
the appropriate inductor current level. To ensure stability,
external frequency compensation components (C
P1
, C
P2
and R
Z
) must be installed between VC and ground. The
procedure for determining these components is provided
in the Applications Information section of this data sheet.
VC is internally connected to the noninverting input of a
high gain, integrating, operational amplifier, referred to
in Figure2, as the average current amp. The inverting
input of the average current amplifier is connected to
the inductor current sense circuit through a gain setting
resistor (R
CS1
) and to its output (VIA) through an internal
frequency compensation network comprised of R
CS2
,
C
CS1
and C
CS2
. The average current amplifier’s output
provides the cycle-by-cycle duty cycle command into the
buck-boost PWM circuitry.
The noninverting reference level input to the average cur
-
rent amplifier is VC and the feedback or inverting input
is driven from the inductor current sensing cir
cuitry. The
inductor current sensing circuitry alternately measures
the current through switches A and B. The output of the
PWM
A B C D
CONTROL
LOGIC
VIA
AVERAGE
CURRENT
AMP
VOLTAGE
ERROR
AMP
ACTIVE RANGE OF
VC = 135mV TO 1V
SOFT-START
1V
R
CS1
C
CS2
R
CS2
C
CS1
R
Z
R
BOT
R
TOP
C
P1
+
–
C
P2
R
COESR
R
LOAD
V
OUT
C
OUT
R
X
g
m
= 111µA/V
V
IN
SWB
SWC
SWD
PV
OUT
SWA
R
A
250mΩ
V
DS
SENSE
L1
V
DS
SENSE
1V
g
m
= 120µS
R
O
= 3.6M
FB
VC
GND
31141 F02
+
–
g
m
INDUCTOR
I
SENSE
Figure2. Average Current Mode Control Loop
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.