ltc3612 Linear Technology Corporation, ltc3612 Datasheet - Page 19

• Manufacturer Part Number: ltc3612
• Description: 3a, 4mhz Monolithic Synchronous Step-down Dc/dc Converter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
3. The TRACK/SS pin can be used to track the output
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
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
tracking state and V
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
again exceeds 0.6V and V
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.
TRACK/SS
value of the internal reference at V
pulled above 0.6V. The external soft-start duration can
be calculated by using the following formula:
voltage of another supply.
t
SS
= R
drops below 0.6V, the LTC3612 enters the down
SS
• C
OUT
SS
• ln
is referenced to the TRACK/SS volt-
⎛
⎝ ⎜
OUT
SV
is referenced to the internal
IN
IN
SV
, once V
– 0.6V
IN
FB
until TRACK/SS is
⎞
⎠ ⎟
TRACK/SS
TRACK/SS
exceeds
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. To implement the ratiometric
tracking behavior in Figure 5b, different resistor divider
values must be used as specifi ed in Figure 6b.
For coincident start-up, the voltage value at the TRACK/SS
pin for the slave channel needs to reach the fi nal reference
value after the internal soft-start time (around 1ms). The
master start-up time needs to be adjusted with an external
capacitor and resistor to ensure this.
Figure 5. Two Different Modes of Output Voltage Tracking
(5b) Ratiometric Tracking
(5a) Coincident Tracking
TIME
TIME
3612 F05
LTC3612
V
V
V
V
OUT1
OUT2
OUT1
OUT2
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3612f