Datasheet

LTM4620A
17
4620afb
For more information www.linear.com/LTM4620A
APPLICATIONS INFORMATION
Figure 5. Operating Frequency vs f
SET
Pin Voltage
The LTM4620A switching frequency can be set with an ex-
ternal resistor from
the f
SET
pin to SGND. An accurate 10µA
current source into the resistor will set a voltage that pro-
grams the
frequency
or a DC voltage can be applied. Figure 5
shows a graph of frequency setting verses programming
voltage.
An external clock can be applied to the MODE/
PLLIN pin from 0V to INTV
CC
over a frequency range of
400kHz to 780kHz. The clock input high threshold is 1.6V
and the clock input low threshold is 1V. The LTM4620A has
the PLL loop filter components on board. The frequency
setting resistor should always be present to set the initial
switching frequency before locking to an external clock.
Both regulators will operate in continuous mode while
being externally clock.
The output of the PLL phase detector has a pair of comple
-
mentary current sources that charge and discharge the
internal filter network. When the external clock is applied
then the f
SET
frequency resistor is disconnected with
an internal switch, and the current sources control the
frequency adjustment to lock to the incoming external
clock. When
no external clock is applied, then the internal
switch is on, thus connecting the external f
SET
frequency
set resistor for free run operation.
Minimum On-Time
Minimum on-time t
ON
is the smallest time duration that
the LTM4620A is capable of turning on the top MOSFET on
either channel. It is determined by internal timing delays,
and the gate charge required turning on the top MOSFET.
Low duty cycle applications may approach this minimum
on-time limit and care should be taken to ensure that:
V
OUT
V
IN
• FREQ
> t
ON(MIN)
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 output ripple will increase. The on-time can be
increased by lowering the switching frequency. A good
rule of thumb is to keep on-time longer than 110ns.
Output Voltage Tracking
Output voltage tracking can be programmed externally
using the TRACK pins. The output can be tracked up
and down with another regulator. The master regulator’s
output is divided down with an external resistor divider
that is the same as the slave regulator’s feedback divider
to implement coincident tracking. The LTM4620A uses
an accurate 60.4k resistor internally for the top feedback
resistor for each channel. Figure 6 shows an example of
coincident tracking. Equations:
SLAVE = 1+
60.4k
R
TA






• V
TRACK
V
TRACK
is the track ramp applied to the slave’s track pin.
V
TRACK
has a control range of 0V to 0.6V, or the internal
reference voltage. When the master’s output is divided
down with the same resistor values used to set the slave’s
output, then the slave will coincident track with the master
until it reaches its final value. The master will continue to
its final value from the slave’s regulation point. Voltage
tracking is disabled when V
TRACK
is more than 0.6V. R
TA
in Figure 6 will be equal to the R
FB
for coincident tracking.
Figure 7 shows the coincident tracking waveforms.
f
SET
PIN VOLTAGE (V)
0
FREQUENCY (kHz)
900
800
600
400
100
200
700
500
300
0
2
4620A F05
2.51 1.50.5