Datasheet

LTC3537
14
3537fd
APPLICATIONS INFORMATION
V
INB
> V
OUTB
OPERATION
The LTC3537 step-up converter will maintain voltage regu-
lation even when the input voltage is above the desired
output voltage. Note that the effi ciency is much lower in this
mode, and the maximum output current capability will be
less. Refer to the Typical Performance Characteristics.
STEP-UP SHORT-CIRCUIT PROTECTION
The LTC3537 output disconnect feature provides output
short circuit protection. To reduce power dissipation under
short-circuit conditions, the peak switch current limit is
reduced to 400mA (typical).
SCHOTTKY DIODE
Although it is not required, adding a Schottky diode from
SW to V
OUTB
will improve effi ciency by about 4%. Note
that this defeats the output disconnect and short-circuit
protection features.
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3537 demands careful
attention to board layout. A careless layout will result in
reduced performance. Figure 1 shows the recommended
component placement. A large ground pin copper area
will help to lower the die temperature. A multilayer board
with a separate ground plane is ideal, but not absolutely
necessary.
COMPONENT SELECTION
Inductor Selection
The LTC3537 can utilize small surface mount chip induc-
tors due to its fast 2.2MHz switching frequency. Inductor
values between 1µH and 4.7µH are suitable for most ap-
plications. Larger values of inductance will allow slightly
greater output current capability (and lower the Burst
Mode threshold) by reducing the inductor ripple current.
Increasing the inductance above 10µH will increase size
while providing little improvement in output current capa-
bility. The minimum inductance value is given by:
L >
V
INB(MIN)
• V
OUTB(MAX)
− V
INB(MIN)
()
2.2 •Ripple•V
OUTB(MAX)
where:
Ripple = Allowable inductor current ripple (amps
peak-peak)
V
INB(MIN)
= Minimum converter input voltage
V
OUTB(MAX)
= Maximum output voltage
The inductor current ripple is typically set for 20% to
40% of the maximum inductor current. High frequency
ferrite core inductor materials reduce frequency dependent
power losses compared to cheaper powdered iron types,
improving effi ciency. The inductor should have low ESR
(series resistance of the windings) to reduce the I
2
R power
losses, and must be able to support the peak inductor
current without saturating. Molded chokes and some chip
inductors usually do not have enough core area to support
the peak inductor currents of 750mA seen on the LTC3537.
To minimize radiated noise, use a shielded inductor. See
Table 1 for suggested components and suppliers.
Figure 1
3537 F01
5678
16 15 14 13
LBO SW
V
OUTB
MODE
V
INL
V
OLDO
LBI
SGND
V
INB
FBL
FBB
1
2
3
4
12
11
10
9
+
PGDB ENBST PGDL ENLDO
MULTIPLE VIAS
TO INNER GROUND LAYERS