LT3992EFE View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT3992EFE' PDF : 36 Pages View PDF
LT3992
Applications Information
Application Optimization
In multiple channel applications requiring large VIN to
VOUT ratios, the maximum frequency and resulting in-
ductor size is determined by the channel with the largest
ratio. The LT3992’s multi-frequency operation allows the
user to minimize component size for each channel while
maintaining constant frequency operation. The circuit in
Figure 15 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 VIN
to VOUT 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 16 steps down the input
voltage (VIN1) to the highest output voltage then uses that
voltage to power the second output (VIN2). VOUT1 must be
able to provide enough current for its output plus VOUT2
maximum load. Note that the VOUT1 voltage must be above
VIN2’s minimum input voltage as specified in the Electrical
Characteristics (typically 2.9V) 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 60V:
VIN = 60V, VOUT1 = 3.3V at 1.5A and VOUT2 = 12V at 1.5A.
Frequency
(Hz) =
VOUT + VD
VIN – VSW + VD
•
1
tON(MIN)
( ) L = VIN – VOUT • VOUT
VIN • f
Single Step-Down:
Frequency
(Hz) =
3.3+ 0.6
60V – 0.4+ 0.6
•1
180ns
≅ 350kHz
L1= (60V – 3.3) • 3.3 ≥ 9µH
60V • 350kHz
L2 = (60V – 12) •12 ≥ 27µH
60V • 350kHz
2-Stage Step-Down:
Frequency
(Hz)
=
12+ 0.6
60V – 0.4+ 0.6
•
1
180ns
≅
1MHz
L1= (60V – 12) •12 ≥ 10µH
60V •1MHz
L2 = (12 – 3.3) • 3.3 ≥ 2.4µH
12 •1MHz
2-Stage Step-Down Multi-Frequency:
RDIV = 61.9k, FREQ1 = 900kHz, FREQ2 = 1800kHz.
L1=
(60V – 12) •12
60V • 900kHz
≥
11µH
L2
=
(12 – 3.3) • 3.3
12 •1800kH z
≥
1.3µH
In addition, RILIM2 = 52.3k reduces the peak current limit
on Channel 2 to 2.5A, which reduces inductor size and
catch diode requirements.
For more information www.linear.com/LT3992
3992fa
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