Datasheet

LTC3816
24
3816f
applicaTions inForMaTion
resistance (0.39%/°C) and produces a near perfect AVP
slope across temperature.
V
OUT
= V
DAC
– A
AVP(DCRP)
• I
L
= V
DAC
– A
G(DCRP)
• I
L
• R
DCR
where:
A A R and A
R
R
AVP DCRP G DCRP DCR G DCRP
VDCRP
LP
( ) ( ) ( )
•= =
TTC
VDCRP
VDCRP DCR
C
L
R R
=
•
I
MON
To facilitate CPU monitoring of load current in an IMVP-6.5
application, the LTC3816 forces the I
MON
pin voltage to
be proportional to the average load current. As shown in
Figure 13, the AITC and the unity-gain amplifiers force
the voltage across the resistor R
PREIMON
to be equal to
the voltage drop across the sense resistor. A current is
supplied to R
IMON
that is three times greater than the cur-
rent in R
PREIMON
. The voltage across the R
IMON
resistor
is equal to:
V I R
R
R
R
R
IMON L SEN
VSR
AVPSR
IMON
PREIMON
=
( )
3• • •
To prevent the ground difference between the CPU and
the regulator from affecting the I
MON
voltage accuracy,
the negative terminal of the resistor R
IMON
should be
connected directly to the CPU V
SS(SEN)
pin. Depending
on the output load requirements, the I
MON
voltage gain
can be programmed by changing the ratio of the R
IMON
and R
PREIMON
resistances. A capacitor should be added
in parallel with the resistor R
IMON
to remove the switching
ripple. The value of the capacitor C
IMON
is determined by
the following equation:
C
t
R
IMON
IMON
IMON
=
where t
IMON
is the I
MON
time constant and must be larger
than 300µs.
In the IMVP-6.5 configuration, the I
MON
pin potential is
internally clamped to 1.1V with respect to the V
SS(SEN)
pin
voltage. Forcing the PREI
MON
pin to INTV
CC
configures the
LTC3816 as an IMVP-6 regulator.
FEEDBACK
CONTROL
The LTC3816 feedback loop consists of the line feed-
forward circuit, the modulator, the external inductor, the
output capacitor, the AITC and differential amplifier, and
the feedback amplifier with its compensation network. All
of these components affect loop behavior and need to be
accounted for in the loop compensation.
Line Feedforward and Modulator
The modulator consists of the PWM generator, the output
MOSFET drivers and the external MOSFETs themselves.
The modulator gain varies linearly with the input voltage.
The line feedforward circuit compensates for this change in
gain and provides a constant gain from the error amplifier
output to the SW node regardless of input voltage. From
a feedback loop point of view, the combination of the line
feedforward circuit and the modulator looks like a linear
voltage transfer function from COMP to the SW node and
has a gain roughly equal to:
A
MOD
≈ 25V/V ≈ 28dB
It has a fairly benign AC behavior at typical loop compen-
sation frequencies with significant phase shift appearing
at half the switching frequency.
+
Q
B
D
L
Q
T
V
IN
ESL
C
OUT
V
OUT
I
L
SENSE RESISTOR
R
SEN
R
AVPSR
TG
BG
BSOURCE
I
TC
I
TCFB
I
SENN
PREI
MON
I
MON
I
MON
3816 F13
V
SS(SEN)
INTV
CC
LTC3816
SW
R
VSR
C
VSR
R
PREIMON
R
IMON
C
IMON
1X
+
–
AITC
Figure 13. I
MON
Configuration