LTC3614 LINER [Linear Technology], LTC3614 Datasheet - Page 19

• Manufacturer Part Number: LTC3614
• Description: 4A, 4MHz Monolithic Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

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
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
When the LTC3614 is enabled by 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
2. If a longer soft-start period is desired, it can be set ex-
circuit. This circuit ramps the output voltage to the fi nal
value within 1ms.
ternally with a resistor and capacitor on the TRACK/SS
pin as shown in Figure 1. The TRACK/SS pin reduces the
V
V
f
SW
IN
OUT
= 3.3V
= 2.25MHz
= 1.8V
IN
Figure 5. Slew Rate at SW Pin vs SRLIM/DDR Resistor: Open, 100k, 10k
selects the internal soft-start
2ns/DIV
10k
100k
Q
OPEN
< 1μA).
IN
, lower
3. The TRACK/SS pin can be used to track the output
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.
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.
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.
V
V
f
t
SW
SS
IN
OUT
= 3.3V
= 2.25MHz
= 1.8V
= R
SS
• C
SS
2ns/DIV
• ln
OPEN
100k
⎛
⎝ ⎜
10k
SV
IN
SV
– 0.6V
IN
FB
3614 F05
until TRACK/SS is
⎞
⎠ ⎟
LTC3614
19
3614f