lt3695 Linear Technology Corporation, lt3695 Datasheet - Page 19

• Manufacturer Part Number: lt3695
• Description: 1a Fault Tolerant Micropower Step-down Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
by using a Zener diode between V
the BD pin voltage between 3V and 8V.
The minimum operating voltage of the LT3695 applica-
tion 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 is turned on with its RUN/SS pin when 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
V
V
(4a) For V
V
IN
IN
IN
(4b) For 2.5V < V
Figure 4. Three Circuits for Generating
the Boost Voltage
(4c) For V
OUT
V
V
V
IN
IN
IN
GND
GND
GND
> 2.8V, V
LT3695
LT3695
LT3695
BD
BD
OUT
BD
PGND
PGND
BOOST
BOOST
PGND
BOOST
OUT
< 2.5V, V
SW
SW
SW
DA
DA
DA
IN(MIN)
< 2.8V, V
IN(MAX)
= 4.3V if V
C3
D1
C3
D1
OUT
C3
D1
IN(MIN)
D2
and BD to maintain
3695 F04a
3695 F04b
3695 F04c
= 25V
= 4.3V
OUT
V
V
V
< 3V
OUT
OUT
OUT
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
For lower start-up voltage, the boost diode can be tied to
V
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. This reduces the
minimum input voltage to approximately 300mV above
V
continuous and the duty cycle is limited by the maximum
duty cycle of the LT3695, requiring a higher input voltage
to maintain regulation.
IN
OUT
; however, this restricts the input range to one-half of
. At higher load currents, the inductor current is
Figure 5. The Minimum Input Voltage depends on
Output Voltage, Load Current and Boost Circuit
4.0
3.5
3.0
2.5
2.0
6.0
5.5
5.0
4.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1
1
TO RUN
TO RUN
(WORST CASE)
(WORST CASE)
TO START
TO START
LOAD CURRENT (mA)
LOAD CURRENT(mA)
10
10
IN
100
100
is ramping very slowly.
V
T
L = 10μH
f = 800kHz
OUT
A
V
T
L = 10μH
f = 800kHz
A
= 25˚C
OUT
= 25˚C
= 3.3V
= 5V
3695 F05
1000
1000
LT3695
19
3695f