lt3480emse-trpbf Linear Technology Corporation, lt3480emse-trpbf Datasheet - Page 15

• Manufacturer Part Number: lt3480emse-trpbf
• Description: 38v, 2a, 2.4mhz Step-down Switching Regulator With 70?a Quiescent Current
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
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 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 LT3480, requiring a higher
input voltage to maintain regulation.
IN
Figure 5. Three Circuits For Generating The Boost Voltage
; however, this restricts the input range to one-half of
4.7μF
4.7μF
4.7μF
V
V
V
IN
IN
IN
(5c) For V
(5b) For 2.5V < V
OUT
V
V
V
IN
IN
IN
. At higher load currents, the inductor
(5a) For V
LT3480
LT3480
LT3480
GND
GND
GND
OUT
BD
BD
BD
BOOST
BOOST
BOOST
< 2.5V; V
SW
SW
SW
OUT
OUT
> 2.8V
IN
IN(MAX)
C3
C3
C3
is ramping very slowly.
< 2.8V
D2
= 30V
3480 FO5
V
OUT
V
V
OUT
OUT
Soft-Start
The RUN/SS pin can be used to soft-start the LT3480, reduc-
ing the maximum input current during start-up. The RUN/SS
pin is driven through an external RC fi lter to create a voltage
ramp at this pin. Figure 7 shows the start-up and shut-down
waveforms with the soft-start circuit. By choosing a large
RC time constant, the peak start-up current can be reduced
to the current that is required to regulate the output, with
no overshoot. Choose the value of the resistor so that it can
supply 20μA when the RUN/SS pin reaches 2.5V.
Synchronization
To select Low-Ripple Burst Mode operation, tie the SYNC
pin below 0.3V (this can be ground or a logic output).
Figure 6. The Minimum Input Voltage Depends on
Output Voltage, Load Current and Boost Circuit
6.0
5.5 TO START
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
1
1
TO RUN
V
T
L = 8.2μH
f = 700kHz
TO RUN
V
T
L = 8.2μH
f = 700kHz
(WORST CASE)
TO START
(WORST CASE)
A
A
OUT
OUT
= 25°C
= 25°C
= 3.3V
= 5V
10
10
LOAD CURRENT (A)
LOAD CURRENT (A)
100
100
1000
1000
3480 F06
10000
10000
LT3480
15
3480fb