Datasheet

LTC3614
19
3614fa
APPLICATIONS INFORMATION
DDR Mode
The LTC3614 can both source and sink current if the MODE
pin is confi gured to forced continuous mode.
Current sinking is typically limited to 3A for 1MHz frequency
and a 0.47µH inductor, but can be lower at higher frequen-
cies and low output voltages. If higher ripple current can
be tolerated, smaller inductor values can increase the sink
current limit. See the Typical Performance Characteristics
curves for more information.
In addition, by tying the SRLIM/DDR pin to SV
IN
, lower
external reference voltage and tracking output voltage are
possible. See the Output Voltage Tracking and External
Reference Input sections.
Soft-Start
The RUN pin provides a means to shut down the LTC3614.
Tying the RUN pin to SGND places the LTC3614 in a low
quiescent current shutdown state (I
Q
< 1µA).
The LTC3614 is enabled by pulling the RUN pin high.
However, the applied voltage must not exceed SV
IN
. In
some applications the RUN signal is generated within
another power domain and is driven high while the SV
IN
and PV
IN
are still 0V. In this case, it’s required to limit
the current into the RUN pin by either adding a 1MΩ
resistor or a 100k resistor plus a Schottky diode to SV
IN
.
After pulling the RUN pin high the chip enters a soft start-up
state. The 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 fi nal
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 LTC3614 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.
Figure 5. Slew Rate at SW Pin vs SRLIM/DDR Resistor: Open, 100k, 10k
2ns/DIV
SW PIN
SW PIN
10k
100k
OPEN
3614 F05
V
IN
= 3.3V
V
OUT
= 1.8V
f
SW
= 2.25MHz
2ns/DIV
V
IN
= 3.3V
V
OUT
= 1.8V
f
SW
= 2.25MHz
10k
100k
OPEN