Input Current Sense Amplifier; Programmable Loop Compensation - Analog Devices Linear ADI Power LTM4681 Manual

APPLICATIONS INFORMATION
to avoid this problem. Multiple LTM4681s are required
to share one SYNC_nn pin in PolyPhase configurations.
For other configurations, connecting the SYNC_nn pins
to form a single SYNC signal is optional. If the SYNC_nn
pin is shared between LTM4681s, only one LTM4681 con-
trollers can be programmed with a frequency output. All
the other LTM4681s should be programmed to disable
the SYNC_nn output. However their frequency should be
programmed to the nominal desired value.

INPUT CURRENT SENSE AMPLIFIER

The LTM4681 input current sense amplifier can sense the
supply current into the V
IN01
using an external sense resistor as shown in the Figure 2
Block Diagram. The R
SENSEn
using the MFR_IIN_CAL_GAIN command. Kelvin sensing
is recommended across the R
errors. The MFR_PWM_CONFIG [6:5] sets the input cur-
rent sense amplifier gain. See the MFR_PWM_CONFIG
section. The IIN_OC_WARN_LIMIT command sets the
value of the input current measured by the ADC, in
amperes, that causes a warning indicating the input cur-
rent is high. The READ_IIN value will be used to determine
if this limit has been exceeded. The READ_IIN command
returns the input current, in Amperes, as measured across
the input current sense resistor.
There is an IR voltage drop from the supply to the SV
pin due to the current flowing into the SV
compensate for this voltage drop, the MFR_RVIN will
be automatically set to the 1Ω internal sense resistor in
the Figure 2 Block Diagram. The LTM4681 will multiply
the MFR_READ_ICHIP measurement value by this 1Ω
resistor and add this voltage to the measured voltage at
the SV pin. Therefore, READ_VIN = VSVIN_PIN +
IN_nn
(MFR_READ_ICHIP • 1Ω) The MFR_READ_ICHIP com-
mand is used to measure the internal controller current.
Using the READ_PIN command allows for reading calcu-
lated input power.

PROGRAMMABLE LOOP COMPENSATION

The LTM4681 offers programmable loop compensation
to optimize the transient response without any hardware
and V power stages pins
IN23
value can be programmed
resistor to eliminate
SENSE
IN_nn
pin. To
IN_nn
For more information
change. The error amplifier gain g
to 5.76mS, and the compensation resistor R
from 0kΩ to 62kΩ inside the controller. Two compensa-
tion capacitors, COMPna and COMPnb, are required in
the design and the typical ratio between COMPn a and
COMPn b is 10. Also see Figure 2 Block Diagram and
Figure 27.
By adjusting the g and R
m
provide a flexible Type II compensation network to opti-
mize the loop over a wide range of output capacitors.
Adjusting the g will change the gain of the compensation
m
over the whole frequency range without moving the pole
and zero location, as shown in Figure 28.
Adjusting the R will change the pole and zero loca-
COMP
tion, as shown in Figure 29. It is recommended that the
user determines the appropriate value for the g
R using the LTPowerCAD tool.
COMPn
R
COMPn
COMPna
C
COMPL
Figure 27. Programmable Loop Compensation
TYPE II COMPENSATION
GAIN
Figure 28. Error Amp g
www.analog.com
LTM4681
varies from 1.0mS
m
COMPn
only, the LTM4681 can
COMPn
+
V
REF
g
m
–
FB
COMPnb
4681 F27
C
COMPH
INCREASE g
FREQUENCY
4681 F28
Adjust
m
63
varies
and
m
m
Rev. A