LTC3614_15 Datasheet, PDF (20/30 Pages) Linear Technology – 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3614
Applications Information
During either internal or external soft-start, the MODE pin
is ignored and soft-start will always be in pulse-skipping
mode. In addition, the PGOOD pin is kept low and foldback
of the switching frequency is disabled.
Programmable Switch Pin Slew Rate
As switching frequencies rise, it is desirable to minimize the
transition time required when switching to minimize power
losses and blanking time for the switch to settle. However,
fast slewing of the switch node results in relatively high
external radiated EMI and high on chip supply transients,
which can cause problems for some applications.
The LTC3614 allows the user to control the slew rate of
the switching node SW by using the SRLIM/DDR pin.
Tying this pin to ground selects the fastest slew rate. The
slowest slew rate is selected when the pin is open. Con-
necting a resistor (between 10k and 100k) from SRLIM
pin to ground adjusts the slew rate between the maximum
and minimum values. The reduced dV/dt of the switch
node results in a significant reduction of the supply and
ground ringing, as well as lower radiated EMI.
Particular attention should be used with very high switching
frequencies. Using the slowest slew rate (SRLIM open)
can reduce the minimum duty cycle capability.
Output Voltage Tracking Input
If the DDR pin is not tied to SV_IN, once V_TRACK/SS exceeds
0.6V, the run state is entered and the MODE selection,
power good and current foldback circuits are enabled.
In the run state, the TRACK/SS pin can be used for track-
ing down/up the output voltage of another supply. If the
V_TRACK/SS drops below 0.6V, the LTC3614 enters the
down tracking state and V_OUT is referenced to the TRACK/
SS voltage. If the TRACK/SS pin drops below 0.2V, the
switching frequency is reduced to ensure that the mini-
mum duty cycle limit does not prevent the output from
following the TRACK/SS pin. The run state will resume if
V_TRACK/SS again exceeds 0.6V and V_OUT is referenced to
the internal precision reference (see Figure 8).
Through the TRACK/SS pin, the output voltage can be set
up for either coincident or ratiometric tracking, as shown
in Figure 6.
V_OUT1
V_OUT2
V_OUT1
V_OUT2
TIME
(6a) Coincident Tracking
TIME
3614 F06
(6b) Ratiometric Tracking
Figure 6. Two Different Modes of Output Voltage Tracking
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