LTC3526L-2_15 Datasheet, PDF (10/16 Pages) Linear Technology – 550mA 2MHz Synchronous Step-Up DC/DC Converters

LTC3526L-2/LTC3526LB-2
Applications Information
V_IN > V_OUT Operation
The LTC3526L-2/LTC3526LB-2 will maintain voltage regu-
lation even when the input voltage is above the desired
output voltage. Note that the efficiency is much lower in this
mode, and the maximum output current capability will be
less. Refer to the Typical Performance Characteristics.
Short-Circuit Protection
The LTC3526L-2/LTC3526LB-2 output disconnect feature
allows output short circuit while maintaining a maximum
internally set current limit. To reduce power dissipation
under short-circuit conditions, the peak switch current
limit is reduced to 400mA (typical).
Schottky Diode
Although not recommended, adding a Schottky diode from
SW to V_OUT will improve efficiency by about 2%. Note
that this defeats the output disconnect and short-circuit
protection features.
PCB layout guidelines
The high speed operation of the LTC3526L-2/LTC3526LB‑2
demands careful attention to board layout. A careless
layout will result in reduced performance. Figure 2 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 LTC3526L-2/LTC3526LB-2 can utilize small surface
mount chip inductors due to their fast 2MHz switching
frequency. Inductor values between 1.5µH and 4.7µH are
suitable for most applications. 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 6.8µH will
increase component size while providing little improvement
in output current capability.
The minimum inductance value is given by:
( ) L > VIN(MIN) • VOUT(MAX) – VIN(MIN)
2 •Ripple • V_OUT(MAX)
where:
Ripple = Allowable inductor current ripple (amps peak-
peak)
V_IN(MIN) = Minimum input voltage
V_OUT(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 depen-
dent power losses compared to cheaper powdered iron
types, improving efficiency. The inductor should have
low ESR (series resistance of the windings) to reduce the
I²R power losses, and must be able to support the peak
10
SW 1
LTC3526L-2
6 V_OUT
GND 2
5 FB
MINIMIZE
TRACE ON FB
AND SW
+
V_IN
V_IN 3
4 SHDN
MULTIPLE VIAS
TO GROUND PLANE
3526lb2 F02
Figure 2. Recommended Component Placement for Single Layer Board
3526lb2fa