Datasheet

LT3724
16
3724fd
APPLICATIONS INFORMATION
The desired soft-start rise time (t
SS
) is programmed via
a programming capacitor C
SS1
, using a value that cor-
responds to 2µA average current during the soft-start
interval. This capacitor value follows the relation:
C
SS1
=
2 • 10
–6
• t
SS
V
OUT
R
SS
is typically set to 200k for most applications.
Considerations for Low-Voltage Output Applications
The LT3724 C
SS
pin biases to 220mV during the soft-start
cycle, and this voltage is increased at Figure 4 node “A” by
the 2µA signal current through R
SS
, so the output has to
reach this value before the soft-start function is engaged.
The value of this output soft-start startup voltage offset
(V
OUT(SS)
) follows the relation:
V
OUT(SS)
= 220mV + R
SS
• 2 • 10
–6
Which is typically 0.64V for R
SS
= 200k.
In some low voltage output applications, it may be desir-
able to reduce the value of this soft-start startup voltage
offset. This is possible by reducing the value of R
SS
. With
reduced values of R
SS
, the signal component caused by
voltage ripple on the output must be minimized for proper
soft-start operation.
Peak-to-peak output voltage ripple (∆V
OUT
) will be imposed
on node “A” through the capacitor C
SS1
. The value of R
SS
can be set using the following equation:
R
SS
=
∆
V
OUT
1.3 • 10
–6
It is important to use low ESR output capacitors for LT3724
voltage converter designs to minimize this ripple voltage
component. A design with an excessive ripple component
can be evidenced by observing the V
C
pin during the start
cycle.
The soft-start cycle should be evaluated to verify that the
reduced R
SS
value allows operation without excessive
modulation of the V
C
pin before finalizing the design.
If V
C
pin has an excessive ripple component during the
soft-start cycle, converter output ripple should be reduced.
This is typically accomplished by increasing output capaci-
tance and/or reducing output capacitor ESR.
External Current Limit Foldback Circuit
An additional startup voltage offset can occur during the
period before the LT3724 soft-start circuit becomes ac-
tive. Before the soft-start circuit throttles back the V
C
pin
in response to the rising output voltage, current as high
as the peak programmed current limit (I
MAX
) can flow in
the switched inductor. Switching will stop once the soft-
start circuit takes hold and reduces the voltage on the
V
C
pin, but the output voltage will continue to increase
as the stored energy in the inductor is transferred to the
output capacitor. With I
MAX
in the inductor, the resulting
leading-edge rise on V
OUT
due to energy stored in the
inductor follows the relation:
∆V
OUT
=I
MAX
•
L
C
OUT






1/2
Figure 6. Desirable Soft-Start Characteristic
Figure 5. Soft-Start Characteristic
Showing Excessive Ripple Component
TIME, 250µs/DIV
V(V
C
)
V
OUT(SS)
V
OUT
3724 F05
TIME, 250µs/DIV
3724F06
V(V
C
)
V
OUT(SS)
V
OUT