Minimum On-Time Considerations; Variable Delay Time, Soft-Start And Output Voltage Ramping; Digital Servo Mode - Analog Devices LTM4683 Manual

LTM4683
APPLICATIONS INFORMATION
The overcurrent fault is detected when the COMPnb volt-
age hits the maximum value. The digital processor within
the LTM4683 can either ignore the fault, shut down and
latch off or shut down and retry indefinitely (hiccup). See
the overcurrent portion of the Operation section for more
detail. The Read_POUT can be used to readback calcu-
lated output power.

MINIMUM ON-TIME CONSIDERATIONS

Minimum on-time, t
ON(MIN)
tion that the LTM4683 is capable of turning on the top
MOSFET. It is determined by internal timing delays and
the gate charge required to turn on the top MOSFET. Low-
duty cycle applications may approach this minimum on-
time limit, and care should be taken to ensure Equation 4.
V
OUTn
t
<
ON(MIN)
V • f
INn OSC
If the duty cycle falls below what can be accommodated
by the minimum on time, the controller will begin to skip
cycles. The output voltage will continue to be regulated,
but the ripple voltage and current will increase.
The minimum on-time for the LTM4683 is 85ns.
VARIABLE DELAY TIME, SOFT-START AND OUTPUT
VOLTAGE RAMPING
The LTM4683 must enter its run state before soft-start.
The RUNn pins are released after the part initializes
and SV exceeds the VIN_ON threshold. If multiple
IN_nn
LTM4683s are used in an application, they should be
configured to share the same RUNn pins. They all hold
their respective RUNn pins low until all devices initialize
and SV exceeds the VIN_ON threshold for all devices.
IN
The SHARE_CLK_nn pin assures that all the devices con-
nected to the signal use the same time base.
After the RUNn pin is released, the controller waits for
the user-specified turn-on delay (TON_DELAYn) before
initiating an output voltage ramp. Multiple LTM4683s and
other Analog Devices ICs can be configured to start with
variable delay times. To work correctly, all devices use the
same timing clock (SHARE_CLK), and all devices must
share the RUNn pin.
58
, is the smallest time dura-
For more information
This allows the relative delay of all parts to be synchro-
nized. The actual variation in the delay will be dependent
on the highest clock rate of the devices connected to
the SHARE_CLK pin (all Analog Devices ICs are config-
ured to allow the fastest SHARE_CLK signal to control
the timing of all devices). The SHARE_CLK signal can
be ±10% in frequency. Thus, the actual time delays will
have some variance.
Soft-start is performed by actively regulating the load volt-
age while digitally ramping the target voltage from 0V to the
commanded voltage set point. The rise time of the voltage
ramp can be programmed using the TON_RISEn command
to minimize inrush currents associated with the start-up
voltage ramp. The soft-start feature is disabled by setting
TON_RISEn to any value less than 0.250ms. The LTM4683
performs the necessary math internally to ensure that the
voltage ramp is controlled to the desired slope. However,
(4)
the voltage slope can not be any faster than the V
damental limits of the power stage. The number of t
steps in the ramp is equal to TON_RISE/0.1ms. Therefore,
the shorter the TON_RISEn time setting, the more discrete
steps in the soft-start ramp appear.
The LTM4683 PWM always operates in discontinuous-
conduction mode during the TON_RISEn operation. In dis-
continuous-conduction mode, the bottom MOSFET (MBn)
is turned off as soon as reverse current is detected in
the inductor. This allows the regulator to start up into a
pre-biased load.
There is no analog tracking feature in the LTM4683; how-
ever, two outputs can be given the same TON_RISEn and
TON_DELAYn times to achieve ratiometric rail tracking.
Because the RUNn pins are released simultaneously, and
both units use the same time base (SHARE_CLK), the
outputs track very closely. If the circuit is in a PolyPhase
configuration, all timing parameters must be the same.

DIGITAL SERVO MODE

For maximum accuracy in the regulated output voltage,
enable the digital servo loop by asserting bit 6 of the
MFR_PWM_MODE command. In digital servo mode, the
LTM4683 will adjust the regulated output voltage based
on the ADC voltage reading. Every 90ms, the digital servo
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fun-
OUTn
ON(MIN)
Rev. 0