Output Current Limit Programming - Analog Devices Linear ADI Power LTM4681 Manual

LTM4681
APPLICATIONS INFORMATION
phasing with respect to the SYNC_nn pin. Phase relation-
ship values are indicated with 0° corresponding to the
falling edge of SYNC being coincident with the turn-on of
the top MOSFETs.
The MFR_PWM_CONFIG command can be altered via
2
I C commands, but only when switching action is dis-
engaged, i.e., the module's outputs are turned off. The
MFR_PWM_CONFIG command takes on the value stored
in NVM at SV power-up, but is overridden according
IN_nn
to a resistor pin-strap applied between the FSWPH_nn_
CFG pin and SGND only if the module is configured to
respect resistor pin-strap settings (MFR_CONFIG_ALL[6]
= 0b). Table 3 highlights available resistor pin-strap and
corresponding MFR_PWM_CONFIG[2:0] settings.
Some combinations of FREQUENCY_SWITCH and
MFR_PWM_CONFIG[2:0] are not available by resistor
pin-strapping the FSWPH_nn_CFG pin. All combinations
of supported values for FREQUENCY_SWITCH and MFR_
PWM_CONFIG[2:0] can be configured by NVM program-
2
ming—or, I C transactions, provided switching action is
disengaged, i.e., the module's outputs are turned off.
Care must be taken to minimize capacitance on SYNC
to assure that the pull-up resistor versus the capacitor
load has a low enough time constant for the application
to form a "clean" clock. (See "Open-Drain Pins", later in
this section.)
When an LTM4681 is configured as a sync slave, it is
permissible for external circuitry to drive the SYNC_nn
pin from a current-limited source (less than 10mA), rather
than using a pull-up resistor. Any external circuitry must
not drive high with arbitrarily low impedance at SV
power-up, because the SYNC_nn output can be low imped-
ance until NVM contents have been downloaded to RAM.
Recommended LTM4681 switching frequencies of oper-
ation for many common V
indicated below. When the two channels of an LTM4681
are stepping input voltage(s) down to output voltages
whose recommended switching frequencies below are
significantly different, operation at the higher of the two
recommended switching frequencies is preferable, but
minimum on-time must be considered. (See Minimum
On-Time Considerations section.)
58
IN_nn
-to-V applications are
IN OUT
For more information
Table 9. Recommended Switching Frequency for Various V
to-V Step-Down Scenarios
OUT
5V
IN
0.9V
OUT
1.0V
OUT
1.2V
OUT
1.5V
OUT
1.8V
OUT
2.5V
OUT
3.3V
OUT

OUTPUT CURRENT LIMIT PROGRAMMING

The cycle-by-cycle current limit (= V
tional to COMPnb, which can be programmed from 1.45V
to 2.2V using the PMBus command IOUT_OC_FAULT_
LIMIT. The LTM4681 uses only the sub-milliohm sensing
to detect current levels. See page 98. The LTM4681
has two ranges of current limit programming. The value
of MFR_PWM_MODE[2] is reserved and the MFR_PWM_
MODE[7], and IOUT_OC_FAULT_LIMIT are used to set
the current limit level, see the section of the PMBus com-
mands, the device can regulate output voltage with the
peak current under the value of IOUT_OC_FAULT_LIMIT
in normal operation. In case of output current exceeding
that current limit, a OC fault will be issued. Each of the
IOUT_OC_FAULT_LIMIT ranges will affect the loop gain,
and subsequently affect the loop stability, so setting the
range of current limiting is a part of loop design.
The LTPowerCAD Design Tool can be used to look at the
loop stability changes if current limit range is adjusted.
The LTM4681 will automatically update the current limit
as the inductor temperature changes. Keep in mind this
operation is on a cycle-by-cycle basis and is only a func-
tion of the peak inductor current. The average inductor
current is monitored by the ADC converter and can provide
a warning if too much average output current is detected.
The overcurrent fault is detected when the COMPnb volt-
age hits the maximum value. The digital processor within
the LTM4681 provides the ability to either ignore the fault,
shut down and latch off or shut down and retry indefinitely
(hiccup). Refer to the overcurrent portion of the Operation
section for more detail. The Read_POUT can be used to
readback calculated output power.
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IN
8V 12V
IN IN
250kHz to 350kHz
425kHz, 500kHz
500kHz, 575kHz
/DCR) is propor-
ISENSE
Rev. A