LTC3816_15 Datasheet, PDF (24/44 Pages) Linear Technology – Single-Phase Wide VIN Range DC/DC Controller for Intel IMVP-6/IMVP-6.5 CPUs

LTC3816
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 _AVP(DCRP) = A_G(DCRP)
• R_DCR
and
A_G(DCRP)
=
R_VDCRP
R_LPTC
C_VDCRP
=
L
R_VDCRP
•
R_DCR
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_IMON ^{= 3 •}
I_L •R_SEN
R_{VSR •} R_IMON
R_AVPSR R_PREIMON
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_IMON
=
t_IMON
R_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.
TG
SW
BG
LTC3816
BSOURCE
INTV_CC
I_TCFB
– ITC
+^AITC
I_SENN
1X
PREI_MON
V_IN
Q_T
L
Q_B
D
C_VSR
R_PREIMON
I_L
ESL R_SEN
SENSE RESISTOR ₊
V_OUT
R_AVPSR
C_OUT
R_VSR
3816 F13
I_MON
R_IMON
C_IMON
I_MON
V_SS(SEN)
Figure 13. I_MON Configuration
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.
3816f
24