Datasheet

LT3692
23
3692fa
For more information www.linear.com/3692
applicaTions inForMaTion
Application Optimization
In multiple channel applications requiring large V
IN
to
V
OUT
ratios, the maximum frequency and resulting in-
ductor size is determined by the channel with the largest
ratio. The LT3692’
s multi-frequency operation allows the
user to minimize component size for each channel while
maintaining constant frequency operation. The circuit in
Figure 17 illustrates this approach. A 2-stage step-down
approach coupled with multi-frequency operation will
further reduce external component size by allowing an
increase in frequency for the channel with the lower V
IN
to V
OUT
ratio. The drawback to this approach is that the
output power capability for the first stage is determined
by the output power drawn from the second stage. The
dual step-down application in Figure 18 steps down the
input voltage (V
IN1
) to the highest output voltage then
uses that voltage to power the second output (V
IN2
). V
OUT1
must be able to provide enough current for its output plus
V
OUT2
maximum load. Note that the V
OUT1
voltage must
be above V
IN2
’s minimum input voltage as specified in the
Electrical Characteristics (2.8V) when the second channel
starts to switch. Delaying channel 2 can be accomplished
by either independent soft-start capacitors or sequencing
with the CMP01 output.
For example, assume a maximum input of 36V:
V
IN
= 36V, V
OUT1
= 3.3V at 1.5A and V
OUT2
= 1.8V at 1.5A.
Frequency (Hz) =
V
OUT
+ V
D
V
IN
– V
SW
+ V
D
•
1
t
ON(MIN)
L =
V
IN
– V
OUT
( )
• V
OUT
V
IN
• f
Single Step Down:
Frequency (Hz) =
1.8 + 0.6
36V – 0.4 + 0.6
•
1
225ns
≅ 250kHz
L1=
36V – 3.3
( )
• 3.3
36V • 250kHz
≥ 12µH
L2 =
36V – 1.8
( )
• 1.8
36V • 250kHz
≥ 6.8µH
2-Stage Step-Down:
Frequency (Hz) =
3.3 + 0.6
36V – 0.4 + 0.6
•
1
225ns
≅ 400kHz
L1=
36V – 3.3
( )
• 3.3
36V • 400kHz
≥ 7.5µH
L2 =
3.3 – 1.8
( )
• 1.8
3.3 • 400kHz
≥ 2µH
2-Stage Step-Down Multi-Frequency:
R
DIV
= 100k, FREQ1 = 400kHz, FREQ2 = 1600kHz.
L1=
36V – 3.3
( )
• 3.3
36V • 400kHz
≥ 7.5µH
L2 =
3.3 – 1.8
( )
• 1.8
3.3 • 2MHz
≥ 500nH
In addition, R
ILIM2
= 40.2k reduces the peak current limit
on channel to 2.5A, which reduces inductor size and catch
diode requirements.