Datasheet

LTC3616
19
3616fc
For more information www.linear.com/LTC3616
applicaTions inForMaTion
In addition, tying the SRLIM/DDR pin to SV
IN
, lower
external reference voltage and tracking output voltage
between channels are possible. See the Output Voltage
Tracking and External Reference Input sections.
Soft-Start
The RUN pin provides a means to shut down the LTC3616.
Tying the RUN pin to SGND places the LTC3616 in a low
quiescent current shutdown state (I
Q
< 1µA).
When the LTC3616 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
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 LTC3616 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.
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.
The LTC3616 allows the user to control the slew rate of
the switching node SW by using the SRLIM/DDR pin.
Tying
this pin to ground selects the fastest slew rate. The
slowest
slew rate is selected when the pin is open. Con-
necting a
resistor (between 10k and 100k) from SRLIM
pin
to ground adjusts the slew rate between the maximum
and minimum values. The reduced dV/dt of the switch
node results in a significant reduction of the supply and
ground ringing, as well as lower radiated EMI.
Particular attention should be used with very high switching
frequencies. Using the slowest slew rate (SRLIM open)
can reduce the minimum duty cycle capability.
Figure 5. Slew Rate at SW Pin vs SRLIM/DDR Resistor: Open, 100k, 10k
2ns/DIV
SW PIN
SW PIN
10k
100k
OPEN
3616 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