LT3693EMSE-PBF LINER [Linear Technology], LT3693EMSE-PBF Datasheet - Page 14

• Manufacturer Part Number: LT3693EMSE-PBF
• Description: 36V, 3.5A, 2.4MHz Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3693
BOOST and BIAS Pin Considerations
Capacitor C3 and the internal boost Schottky diode (see
the Block Diagram) are used to generate a boost volt-
age that is higher than the input voltage. In most cases
a 0.47μF capacitor will work well. Figure 2 shows three
ways to arrange the boost circuit. The BOOST pin must be
more than 2.3V above the SW pin for best effi ciency. For
outputs of 3V and above, the standard circuit (Figure 4a)
is best. For outputs between 2.8V and 3V, use a 1μF boost
capacitor. A 2.5V output presents a special case because it
14
Figure 4. Three Circuits For Generating The Boost Voltage
4.7 F
4.7 F
4.7 F
V
V
V
IN
IN
IN
(4c) For V
(4b) For 2.5V < V
V
V
V
IN
IN
IN
(4a) For V
LT3693
LT3693
LT3693
GND
GND
GND
OUT
BD
BD
BD
BOOST
BOOST
BOOST
< 2.5V; V
SW
SW
SW
OUT
OUT
> 2.8V
IN(MAX)
C3
C3
C3
< 2.8V
D2
= 28V
3693 FO5
V
OUT
V
V
OUT
OUT
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BOOST pin
operation with 2.5V outputs use a good external Schottky
diode (such as the ON Semi MBR0540), and a 1μF boost
capacitor (see Figure 4b). For lower output voltages the
boost diode can be tied to the input (Figure 4c), or to
another supply greater than 2.8V. Tying BD to V
the maximum input voltage to 28V. The circuit in Figure 4a
is more effi cient because the BOOST pin current and BD
pin quiescent current comes from a lower voltage source.
You must also be sure that the maximum voltage ratings
of the BOOST and BD pins are not exceeded.
Figure 5. 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 = 8.2 H
f = 600kHz
TO RUN
V
T
L = 8.2 H
f = 600kHz
(WORST CASE)
TO START
(WORST CASE)
A
A
OUT
OUT
= 25 C
= 25 C
= 3.3V
= 5V
10
10
LOAD CURRENT (mA)
LOAD CURRENT (mA)
100
100
1000
1000
3693 F06
10000
10000
IN
reduces
3693f