Datasheet

LT3501
23
3501fd
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
4.7µF
47µF
42.3k 100k
26.7k
PMEG4005
8.06k
PMEG4005
10pF
470pF
3.3µH
1µH
V
OUT1
5V
V
OUT2
1.2V
V
IN
6V TO 24V
0.47µF
47µF
×2
0.47µF
10pF
B360A
B360A
V
IN1
V
IN2
LT3501
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
3501 F10
32.4k
0.1µF 0.1µF
470pF
applicaTions inForMaTion