Table of Contents
Advertisement
APPLICATIONS INFORMATION
MINIMUM ON-TIME CONSIDERATIONS
Minimum on-time, t
ON(MIN)
that the LTM4700 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 that:
V
OUTn
t
<
ON(MIN)
V
• f
INn
SW
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 LTM4700 is 60ns.
VARIABLE DELAY TIME, SOFT-START AND OUTPUT
VOLTAGE RAMPING
The LTM4700 must enter its run state prior to soft-start.
The RUNn pins are released after the part initializes and
SV
is greater than the VIN_ON threshold. If multiple
IN
LTM4700s 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 pin assures all the devices connected to
the signal use the same time base.
After the RUNn pin releases, the controller waits for the
user-specified turn-on delay (TON_DELAYn) prior to ini-
tiating an output voltage ramp. Multiple LTM4700s and
other ADI parts 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.
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 configured
to allow the fastest SHARE_CLK signal to control the tim-
ing of all devices). The SHARE_CLK signal can be ±10%
, is the smallest time duration
For more information
in frequency, thus the actual time delays will have some
variance.
Soft-start is performed by actively regulating the load
voltage 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 LTM4700 performs the necessary math internally to
assure the voltage ramp is controlled to the desired slope.
However, the voltage slope can not be any faster than the
V
fundamental limits of the power stage. The number of
OUTn
t
< steps in the ramp is equal to TON_RISE/0.1ms.
ON(MIN)
Therefore, the shorter the TON_RISEn time setting, the
more discrete steps in the soft-start ramp appear.
The LTM4700 PWM always operates in discontinuous
mode during the TON_RISEn operation. In discontinuous
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 LTM4700; 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 at the same time 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
LTM4700 will adjust the regulated output voltage based
on the ADC voltage reading. Every 90ms the digital servo
loop will step the LSB of the DAC (nominally 1.375mV or
0.6875mV depending on the voltage range bit) until the
output is at the correct ADC reading. At power-up this mode
engages after TON_MAX_FAULT_LIMIT unless the limit
is set to 0 (infinite). If the TON_MAX_FAULT_LIMIT is set
to 0 (infinite), the servo begins after TON_RISE is complete
and V
has exceeded the VOUT_UV_FAULT_LIMIT.
OUT
www.analog.com
LTM4700
Rev. B
53
Advertisement
Table of Contents
