Switching Frequency And Phase; Pwm Loop Compensation; Output Voltage Sensing; Intv Cc /Vbias Power - Analog Devices Linear ADI Power LTM4681 Manual

OPERATION
the efficiency at light loads is lower than in discontinuous
mode operation. However, continuous mode exhibits lower
output ripple and less interference with audio circuitry, but
may result in reverse inductor current, which can cause
the input supply to boost. The VIN_OV_FAULT_LIMIT can
detect this and turn off the offending channel. However,
this fault is based on an ADC read and can take up to t
to detect. If there is a concern about the input supply
VERT
boosting, keep the part in discontinuous conduction mode.
If the part is set to discontinuous mode operation, as
the inductor average current increases, the controller will
automatically modify the operation from discontinuous
mode to continuous mode.

SWITCHING FREQUENCY AND PHASE

The switching frequency of the PWM can be established
with an internal oscillator or an external time base. The
internal phase-locked loop (PLL) synchronizes the PWM
control to this timing reference with proper phase relation,
whether the clock is provided internally or externally. The
device can also be configured to provide the master clock
to other devices through PMBus command, NVM setting,
or external configuration resistors as outlined in Table 3.
As clock master, the LTM4681 will drive its open-drain
SYNC_nn pin at the selected rate with a pulse width of
500ns. An external pull-up resistor between SYNC_nn
and V is required in this case. Only one device
DD33_nn
connected to SYNC_nn should be designated to drive the
pin. The LTM4681 will automatically revert to an external
SYNC_nn input, disabling its own SYNC_nn, as long as
the external SYNC_nn frequency is greater than 80% of
the programmed SYNC_nn frequency. The external SYNC
input shall have a duty cycle between 20% and 80%.
Whether configured to drive SYNC_nn or not, the LTM4681
can continue PWM operation using its own internal oscil-
lator if an external clock signal is subsequently lost.
The device can also be programmed to always require an
external oscillator for PWM operation by setting bit 4 of
MFR_CONFIG_ALL. The status of the SYNC driver circuit
is indicated by bit 10 of MFR_PADS.
The MFR_PWM_CONFIG command can be used to con-
figure the phase of each channel. Desired phase can also
be set from EEPROM or external configuration resistors
as outlined in Table 3. Designated phase is the relation-
ship between the falling edge of SYNC and the internal
clock edge that sets the PWM latch to turn on the top
power switch. Additional small propagation delays to the
PWM control pins will also apply. Both channels must be
off before the FREQUENCY_SWITCH and MFR_PWM_
CON-
CONFIG commands can be written to the LTM4681.
The phase relationships and frequency options provide for
numerous application options. Multiple LTM4681 mod-
ules can be synchronized to realize a PolyPhase array.
In this case the phases should be separated by 360/n
degrees, where n is the number of phases driving the
output voltage rail.

PWM LOOP COMPENSATION

The internal PWM loop compensation resistors R
of the LTM4681 can be adjusted using bit[4:0] of the
MFR_PWM_COMP command for each controller.
The transconductance (gm) of the LTM4681 PWM error
amplifier can be adjusted using bit[7:5] of the MFR_
PWM_COMP command. These two loop compensation
parameters can be programmed when device is in opera-
tion. Refer to the Programmable Loop Compensation sub-
section in the Applications Information section for further
details.

OUTPUT VOLTAGE SENSING

All four channels in LTM4681 have differential amplifi-
ers, which allow the remote sensing of the load voltage
between V
differential and makes measurements between V
and V
OSNSn
pins, respectively. The maximum allowed 3.6V, but the
LTM4681 design is limited to 3.3V.
INTV /V
CC BIAS
Power for the internal top and bottom MOSFET drivers and
most other internal circuitry is derived from the INTV
pin. When the RUNP pin is shorted to GND and the V
is off, an internal 5.5V linear regulator supplies INTV
For more information www.analog.com
+ –
and V pins. The telemetry ADC is also fully
-voltages for both channels at the V
POWER
LTM4681
COMPna
+
OSNSn
+ –
and V
CC
BIAS
CC
Rev. A
33