Datasheet

LT3988
17
3988f
applicaTions inForMaTion
Independent Input Voltages
V
IN1
and V
IN2
are independent and can be powered with
different voltages provided V
IN1
is present when V
IN2
is
present. Each supply must be bypassed as close to the V
IN
pins as possible. For applications requiring large inductors
due to high V
IN
to V
OUT
ratios, a 2-stage step-down ap-
proach may reduce inductor size by allowing an increase
in frequency. A dual step-down application steps down
the input voltage (V
IN1
) to the highest output voltage, then
uses that voltage to power the other output (V
IN2
). V
OUT1
must be able to provide enough current for its output plus
Figure 11. Subtracting the Current When the Switch Is ON (11a) From the Current When the Switch Is OFF (11b) Reveals the Path of
the High Frequency Switching Current (11c). Keep this Loop Small. The Voltage on the SW and Boost Nodes Will Also Be Switched;
Keep These Nodes as Small as Possible. Finally, Make Sure the Circuit Is Shielded with a Local Ground Plane
the input current at V
IN2
when V
OUT2
is at maximum load.
Figure 10 shows a 12V to 5V, and 1.8V 2-stage converter
using this approach.
PCB Layout
For proper operation and minimum EMI, care must be
taken during printed circuit board (PCB) layout. Figure 11
shows the high current paths in the step-down regula-
tor circuit. Note that in the step-down regulators large,
switched currents flow in the power switch, the catch
diode and the input capacitor. The loop formed by these
Figure 10. 1MHz, 2-Stage Step-Down 5V and 1.8V Outputs
2200pF
57.6k 14k
6.8µH
V
IN
12V
V
OUT1
V
OUT1
5V, 500mA
V
OUT2
1.8V, 500mA
0.22µF
10µF
3988 F10
10.2k
2200pF
3.3µH
0.22µF
22µF
10k
FB1
DA1
SW1
BOOST1
SYNC
TRACK/SS1
FB2
DA2
SW2
BOOST2
BD
TRACK/SS2
EN/UVLO
GND
V
IN1
V
IN2
LT3988
40.2k
RT
4.7µF 4.7µF
V
IN
SW
GND
(11a)
V
IN
V
SW
C1 D1 C2
3988 F11
L1
SW
GND
(11c)
V
IN
SW
GND
(11b)
I
C1