LTC3614_15 Datasheet, PDF (19/30 Pages) Linear Technology – 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter

LTC3614
Applications Information
DDR Mode
The LTC3614 can both source and sink current if the MODE
pin is configured 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 final
value within 1ms.
2. If a longer soft-start period is desired, it can be set
externally 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.
10k
100k
OPEN
OPEN
100k
10k
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
2ns/DIV
3614 F05
Figure 5. Slew Rate at SW Pin vs SRLIM/DDR Resistor: Open, 100k, 10k
For more information www.linear.com/LTC3614
3614fc
19