Datasheet

LTC3612
20
3612fb
For more information www.linear.com/LTC3612
APPLICATIONS INFORMATION
the RUN pin by either adding a 1MΩ resistor or a 100kΩ
resistor, plus a Schottky diode, to SV
IN
. After pulling the
RUN pin high, the chip enters a soft start-up state. This
type of soft start-up behavior is set by the TRACK/SS pin:
1. Tying TRACK/SS to SV
IN
selects the internal soft-start
circuit. This circuit ramps the output voltage to the final
value within 1ms.
2. If a longer soft-start period is desired, it can be set ex
-
ternally with a resistor and capacitor on the TRACK/SS
pin, as shown in Figure 1. The TRACK/SS pin reduces
the value of the internal reference at V
FB
until TRACK/
SS is pulled above 0.6V. The external soft-start duration
can be calculated by using the following formula:
t
SS
= R
SS
•C
SS
•ln
SV
IN
SV
IN
– 0.6V






3. The TRACK/SS pin can be used to track the output
voltage of another supply.
Each time the RUN pin is tied high and the LTC3612 is
turned on, the TRACK/SS pin is internally pulled down
for ten microseconds in order to discharge the external
capacitor. This discharging time is typically adequate for
capacitors up to about 33nF. If a larger capacitor is required,
connect the external soft-start resistor to the RUN pin.
Regardless of either internal or external soft-start state,
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.
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 LTC3612 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
Figure 5. Two Different Modes of Output Voltage Tracking
TIME
(5b) Ratiometric Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
TIME
3612 F05
(5a) Coincident Tracking
V
OUT1
V
OUT2
OUTPUT VOLTAGE
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 7).
Through the TRACK/SS pin, the output voltage can be set
up for either coincident or ratiometric tracking, as shown
in Figure 5.
To implement the coincident tracking behavior in Fig-
ure
5a, connect an extra resistive divider to the output
of the master channel and connect its midpoint to the
TRACK/SS pin for the slave channel. The ratio of this
divider should be selected to be the same as that of the
slave channel’s feedback divider (Figure 6a).
In this track-
ing mode, the master channel’s output must be set higher
than slave channel’s output. T
o implement the ratiometric
tracking behavior in Figure 5b, different resistor divider
values must be used as specified in Figure 6b.