Datasheet
LT3690
19
3690fa
applicaTions inForMaTion
is also limited by the boost circuit. If the input voltage is
ramped slowly, or the LT3690 is turned on with its EN pin
when the output is already in regulation, then the boost
capacitor may not be fully charged. Because the boost
capacitor charges with the energy stored in the inductor,
the circuit relies on some minimum load current to get the
boost circuit running properly. This minimum load depends
on the input and output voltages, and on the arrangement
of the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 6 shows a plot of
minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher, which will allow it to
start. The plots show the worst-case situation, where V
IN
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V
IN
; however, this restricts the
input range to one-half of the absolute maximum rating of
the BST pin. At light loads, the inductor current becomes
discontinuous and the effective duty cycle can be very high.
This reduces the minimum input voltage to approximately
300mV
above V
OUT
. At higher load currents, the inductor
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3690, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SS (soft-start) pin provides a soft-start function. If a
capacitor C
SS
is tied from the SS pin to ground, then the
internal pull-up current will generate a voltage ramp on this
pin. A good value for the soft-start capacitor is C
OUT
/10000,
where C
OUT
is the value of the output capacitor.
The soft-start function limits peak input current to the circuit
during start-up. The output of the LT3690 regulates to the
lowest voltage present at either the SS pin or an internal
0.8V reference. A capacitor from the SS pin to ground is
charged by an internal 2μA current source resulting in a
linear output ramp from 0V to the regulated output volt-
age. The ramp duration is given by:
t
RAMP
=
C
SS
• 0.8V
2µA
At power-up, an internal open-collector output discharges
the SS pin. The SS pin can be left floating if the soft-start
feature is not used. The internal current sources will charge
this pin to ~2V as shown in Figure 7.
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
Figure 7. Soft-Start Ramp
50ms/DIV
V
SS
1V/DIV
V
EN
2V/DIV
I
L
2A/DIV
C
SS
= 0.22µF
V
OUT
2V/DIV
3690 F07
LOAD CURRENT (mA)
1
INPUT VOLTAGE (V)
6.0
3.5
5.5
5.0
4.0
4.5
3.0
10010 1000
3690 F06a
10000
TO RUN
TO START
V
OUT
= 3.3V
L = 4.7µH
ƒ = 600kHz
LOAD CURRENT (mA)
1
INPUT VOLTAGE (V)
7.5
5.0
7.0
5.5
6.5
6.0
4.5
10010 1000
3690 F06b
10000
TO RUN
TO START
V
OUT
= 5V
L = 4.7µH
ƒ = 600kHz