lt3684emse-trpbf Linear Technology Corporation, lt3684emse-trpbf Datasheet - Page 15

• Manufacturer Part Number: lt3684emse-trpbf
• Description: 36v, 2a, 2.8mhz Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
proper start-up, the minimum input voltage is also limited
by the boost circuit. If the input voltage is ramped slowly,
or the LT3684 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 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
Figure 4. Three Circuits For Generating The Boost Voltage
4.7µF
4.7µF
4.7µF
V
V
V
IN
IN
IN
(4b) For 2.5V < V
V
V
V
IN
IN
IN
(4a) For V
(4c) For V
LT3684
LT3684
LT3684
GND
GND
GND
BD
BD
BD
BOOST
BOOST
BOOST
SW
SW
SW
OUT
OUT
OUT
> 2.8V
< 2.5V
C3
C3
C3
< 2.8V
D2
3684 FO4
V
OUT
V
V
OUT
OUT
voltage. In many cases the discharged output capacitor
will present a load to the switcher and the minimum input
to start will be the same as the minimum input to run.
This occurs, for example, if RUN/SS is asserted after V
is applied. The plots show the worst-case situation where
V
boost diode can be tied to V
input range to one-half of the absolute maximum rating
of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous 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 LT3684, requiring a higher
input voltage to maintain regulation.
IN
is ramping very slowly. For lower start-up voltage, the
Figure 5. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
0.001
0.001
TO RUN
TO RUN
TO START
TO START
OUT
. At higher load currents, the inductor
0.01
0.01
LOAD CURRENT (A)
LOAD CURRENT (A)
0.1
0.1
IN
; however, this restricts the
V
T
L = 4.7µH
f = 800kHz
V
T
L = 4.7µH
f = 800kHz
A
1
1
OUT
A
OUT
= 25°C
= 25 °C
= 3.3V
= 5V
3684 F05
10
10
LT3684
15
3684f
IN