Datasheet

LT3510
23
3510fe
inductor size by allowing an increase in frequency. A dual
step down application (Figure 10) 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
must be above V
IN2
minimum input voltage (2V) when the second channel
starts to switch. Delaying channel 2 can be accomplished
by either independent soft-start capacitors or sequencing
with the PG1 output.
For example, assume a maximum input of 24V:
V
IN
= 24V, V
OUT1
= 5V at 1.5A and V
OUT2
= 1.2V at 1.5A
Frequency (Hz) ≤
V
OUT
+ V
D
V
IN
–V
SW
+ V
D
t
MIN(ON)
L ≥
V
IN
–V
OUT
()
•V
OUT
V
IN
•f
Figure 10. 5V and 1.2V 2-Stage Step-Down Converter with Output Sequencing
Single Step Down:
Frequency (Hz) ≤
1.2+ 0.6
24 – 0.4 + 0.6
190ns
= 392kHz
L1=
24 – 5
()
•5
24 • 392kHz
≥ 10μH
L2 =
24 – 1.2
()
•1.2
24 • 392kHz
≥ 2.7μH
2-Stage Step-Down:
Frequency ≤
5 + 0.6
24 – 0.4 + 0.6
190ns
= 1.2MHz
Max Frequency = 1.2MHz
L1=
24 – 5
()
•5
24 •1.2MHz
≥ 3.3μH
L2 =
5 – 1.2
()
•1.2
5 •1.2MHz
≥ 0.76μH
SHDN
4.7
μF
47μF
42.3k 100k
26.7k
PMEG4005
8.06k
PMEG4005
10pF
470pF
3.3
μH
1
μH
V
OUT1
5V
V
OUT2
1.2V
V
IN
6V TO 24V
0.47
μF
47μF
s2
0.47
μF
10pF
B360A
B360A
V
IN1
V
IN2
LT3510
GND
BST1
SW1
IND1
V
OUT1
PG1
FB1
V
C1
SS/TRACK1
FSET
BST2
SW2
IND2
V
OUT2
PG2
FB2
V
C2
SS/TRACK2
40.2k
4k
8.06k
3510 F10
32.4k
0.1μF 0.1μF
470pF
APPLICATIONS INFORMATION