LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 19

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

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
is ramping very slowly. For lower start-up voltage, the
boost diode can be tied to V
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
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
internal pull-up current will generate a voltage ramp on this
6.0
5.5
5.0
4.5
4.0
3.5
3.0
SS
1
TO RUN
is tied from the SS pin to ground, then the
TO START
OUT
Figure 6. The Minimum Input Voltage Depends on Output Voltage, Load Current and Boost Circuit
. At higher load currents, the inductor
10
LOAD CURRENT (mA)
100
IN
; however, this restricts the
1000
V
L = 4.7µH
ƒ = 600kHz
OUT
= 3.3V
3690 F06a
10000
IN
pin. A good value for the soft-start capacitor is C
where C
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 voltage.
The ramp duration is given by:
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.
t
RAMP
OUT
2V/DIV
1V/DIV
2V/DIV
2A/DIV
=
V
V
V
OUT
7.5
7.0
6.5
6.0
5.5
5.0
4.5
EN
SS
I
C
L
is the value of the output capacitor.
1
SS
C
SS
2µA
TO RUN
• 0.8V
= 0.22µF
Figure 7. Soft-Start Ramp
TO START
10
LOAD CURRENT (mA)
50ms/DIV
100
1000
V
L = 4.7µH
ƒ = 600kHz
OUT
= 5V
3690 F06b
3690 F07
10000
LT3690
OUT
/10000,
19
3690f