Output Current Sensing And Sub Milliohm Dcr Current Sensing; Input Current Sensing; Polyphase Load Sharing - Analog Devices Linear ADI Power LTM4681 Manual

LTM4681
OPERATION
power from SV . If V
IN_nn BIAS
is higher than 7.0V, the 5.5V regulator is turned off and
an internal switch is turned on, connecting V
the V allows the INTV
BIAS
high efficiency internal source. V
to the internal 3.3V linear regulators when V
which allows the LTM4681 controllers to be initialized and
programmed even with channels off.
The INTV regulator is powered from the SV
CC_nn
the power through the IC is equal to SV
The gate charge current is dependent on operating fre-
quency. The INTV regulator can supply up to 100mA,
CC_nn
and the typical INTV
CC_nn
~50mA. A 12V input voltage would equate to a difference
of 7V per controller drop across the internal controller,
when multiplied by 50mA equals a 350mW power loss.
This loss can be eliminated by ultilizing the V
Do not tie INTV on the LTM4681 to an external sup-
CC_nn
ply because INTV will attempt to pull the external
CC_nn
supply high and hit current limit, significantly increasing
the die temperature.
For applications where V
INTV pins together to the 5V input through a 1Ω
CC_nn
resistor as shown in Test Circuit 2.
OUTPUT CURRENT SENSING AND SUB MILLIOHM
DCR CURRENT SENSING
The LTM4681 use a unique sub-milliohm inductor cur-
rent sensing technique that provides a high level signal
to noise ratio while sensing very low signals in current
mode operation. This enables higher conversion efficien-
cies with the use of the internal sub-milliohm inductors in
heavy load applications. The current limit threshold can
be accurately set with the MFR_PWM_MODE[7] for High
and Low range (see page 98).
The internal DCR sensing network, thus current limit are
calculated based on the DCR of the inductor at room tem-
perature. The DCR of the inductor has a large temperature
coefficient, approximately 3900ppm/°C. The temperature
coefficient of the inductor is written to the MFR_IOUT_
CAL_GAIN_TC register. The external temperature is sensed
near the inductor and used to modify the internal current
34
is on at 5.5V output and V
. Using
BIAS
power to be derived from a
CC
can provide power
BIAS
is present,
IN
IN_nn
• I
IN_nn INTVCCnn
current for the LTM4681 is
regulator.
BIAS
is 5V, tie the SV
IN IN_nn
For more information
limit circuit to maintain an essentially constant current
IN
limit with temperature. The current sensed is then digitized
by the LTM4681's telemetry ADC with an input range of
±128mV, a noise floor of 7µV
approximately 46.5µV. The LTM4681 computes the induc-
tor current using the DCR value stored in the IOUT_CAL_
GAIN command and the temperature coefficient stored in
command MFR_IOUT_CAL_GAIN_TC. The resulting cur-
rent value is returned by the READ_IOUT command.
pin,
.

INPUT CURRENT SENSING

To sense the total input current consumed by the
LTM4681's power stages , a sense resistor is placed
between the supply voltage and the drain of the top
N-channel MOSFET. The I
nected to the sense resistor. The filtered voltage is ampli-
fied by the internal high side current sense amplifier and
digitized by the LTM4681's telemetry ADC. The input cur-
rent sense amplifier has three gain settings of 2x, 4x, and
8x set by the bit[6:5] of the MFR_PWM_CONFIG com-
mand. The maximum input sense voltage for the three
gain settings is 50mV, 25mV, and 10mV respectively. The
and
LTM4681 computes the input current using the internal
R value stored in the IIN_CAL_GAIN command. The
SENSE
resulting measured power stage current is returned by
the READ_IIN command. I
1 (channel 0 and 1), and I
(channel 2 and 3).
The LTM4681 uses a 1Ω resistor to measure the SV
pin supply current being consumed by each LTM4681
internal controller. This value is returned by the MFR_
READ_ICHIP command. The chip current is calculated by
using the 1Ω value stored in the MFR_ICHIP_CAL_GAIN
command. Refer to the subsection titled Input Current
Sense Amplifier in the Applications Information section
for further details.

PolyPhase LOAD SHARING

Multiple LTM4681s can be arrayed in order to provide a
balanced load-share solution by bussing the necessary
pins. Figure 50 illustrates a 8-Phase design sharing con-
nections required for load sharing.
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, and a peak-peak noise of
RMS
+ –
and I pins are con-
IN_nn IN_nn
+ –
, I for controller
IN_01 IN_01
+ –
, I for controller 2
IN_23 IN_23
IN_nn
Rev. A