LTM4620 Datasheet, PDF (16/36 Pages) Linear Technology – Dual 13A or Single 26A DC/DC μModule Regulator

LTM4620
Applications Information
900
800
700
600
500
400
300
200
100
0
0
0.5
1
1.5
2
f_SET PIN VOLTAGE (V)
2.5
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Figure 5. Operating Frequency vs f_SET Pin Voltage
The LTM4620 switching frequency can be set with an
external resistor from the f_SET pin to SGND. An accurate
10µA current source into the resistor will set a voltage
that programs the frequency or a DC voltage can be ap-
plied. 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 thresh-
old is 1.6V and the clock input low threshold is 1V. The
LTM4620 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 clocked.
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,
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 LTM4620 is capable of turning on the top MOSFET on
either channel. 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_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 LTM4620 uses an
accurate 60.4k resistor internally for the top feedback
resistor for each channel. Figure 6 shows an example of
coincident tracking.
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.
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