Datasheet

LTC3614
20
3614fa
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 slow-
est slew rate is selected when the pin is open. Connecting
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 signifi cant reduction of the supply and ground
ringing, as well as lower radiated EMI.
Figure 6. Two Different Modes of Output Voltage Tracking
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 volt-
age. If the TRACK/SS pin drops below 0.2V, the switching
frequency is reduced to ensure that the minimum 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.
TIME
(6b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3614 F06
(6a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE