Linear Technology LT3695 Series, Datasheet

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LT3695 Series
19
3695fa
the output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on 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 5 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
BOOST pin. At light loads, the inductor current becomes
discontinuous and the effective duty cycle can be very high.
APPLICATIONS INFORMATION
BD
LT3695
V
IN
V
IN
C3
D1
V
OUT
3695 F04a
BOOST
SW
DA
GND PGND
(4a) For V
OUT
> 2.8V, V
IN(MIN)
= 4.3V if V
OUT
< 3V
BD
LT3695
V
IN
V
IN
C3
D1
D2
V
OUT
3695 F04b
BOOST
SW
DA
GND PGND
(4b) For 2.5V < V
OUT
< 2.8V, V
IN(MIN)
= 4.3V
BD
LT3695
V
IN
V
IN
C3
D1
V
OUT
3695 F04c
BOOST
SW
DA
GND PGND
(4c) For V
OUT
< 2.5V, V
IN(MAX)
= 25V
Figure 4. Three Circuits for Generating
the Boost Voltage for the LT3695
the circuit in Figure 4a. For higher output voltages, make
sure that there is no more than 8V at the BD pin either by
connecting it to another available supply higher than 3V or
by using a Zener diode between V
OUT
and BD to maintain
the BD pin voltage between 3V and 8V.
The minimum operating voltage of the LT3695 regulators
application is limited by the minimum input voltage and by
the maximum duty cycle as outlined previously. For proper
start-up, the minimum input voltage is also limited by the
boost circuit. If the input voltage is ramped slowly, or the
LT3695 regulators are turned on with their RUN/SS pin when
LOAD CURRENT (mA)
INPUT VOL
TAGE (V)
1
4.5
5.0
5.5
1000
3.5
4.0
3.0
2.5
2.0
10 100
10 100
6.0
LOAD CURRENT(mA)
INPUT VOL
TAGE (V)
3695 F05
1
6.0
6.5
7.0
7.5
1000
4.5
4.0
5.5
5.0
3.5
2.5
3.0
2.0
8.0
TO RUN
TO START
(WORST CASE)
TO START
(WORST CASE)
V
OUT
= 3.3V
T
A
= 25˚C
L = 10μH
f = 800kHz
TO RUN
V
OUT
= 5V
T
A
= 25˚C
L = 10μH
f = 800kHz
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit