LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 18

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3690
The switching will only resume once the low side switch
current has fallen below the 5A limit. This way, the com-
parator regulates the valley current of the inductor to
5A during short-circuit. With properly chosen external
components, this will ensure that the part will survive a
short-circuit event.
V
The linear voltage regulator requires a capacitor of 0.47µF
to deliver the peak current for the gate driver of the low
side N-channel transistor. The output voltage is monitored
by a comparator. To ensure proper operation, the low side
driver only turns on if V
BST and BIAS Pin Considerations
Capacitor C
the Block Diagram) are used to generate boost voltages
that are higher than the input voltage. In most cases a
0.68µF capacitor will work well. Figure 5 shows three
ways to arrange the boost circuit. The BST pin must be
18
CCINT
V
IN
Considerations
BST
and the internal boost Schottky diode (see
V
(5b) 2.5V < V
IN
LT3690
GND
BIAS
CCINT
BST
SW
OUT
is above 3.8V (typ).
D2
< 2.8V
Figure 5. Three Circuits for Generating the Boost Voltage
C
BST
V
IN
3690 F05b
V
IN
V
LT3690
OUT
GND
(5a) V
BIAS
BST
SW
OUT
C
more than 2.3V above the SW pin for best efficiency. For
outputs of 3V and above, the standard circuit (Figure 5a)
is best. For outputs between 2.8V and 3V, use a 1µF boost
capacitor. A 2.5V output presents a special case because it
is marginally adequate to support the boosted drive stage
while using the internal boost diode. For reliable BST 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 5b). For lower output voltages, the
boost diode can be tied to the input (Figure 5c), or to
another supply greater than 2.8V. The circuit in Figure 5a
is more efficient because the BST pin current and BIAS
pin quiescent current comes from a lower voltage source.
However, the full benefit of the BIAS pin is not realized
unless it is at least 3V. Ensure that the maximum voltage
ratings of the BST and BIAS pins are not exceeded.
The minimum operating voltage of an LT3690 application
is limited by the minimum input voltage (3.9V) and by the
maximum duty cycle as outlined in the Input Voltage Range
section. For proper start-up, the minimum input voltage
BST
> 2.8
V
IN
3690 F05a
(5c) V
V
OUT
V
IN
LT3690
OUT
GND
BIAS
< 2.5V, V
BST
SW
C
BST
IN(MAX)
= 27V
3690 F05c
V
OUT
3690f