LTC3526B_15 Datasheet, PDF (10/16 Pages) Linear Technology – 500mA 1MHz Synchronous Step-Up DC/DC Converters

LTC3526/LTC3526B
Applications Information
V_IN > V_OUT Operation
The LTC3526/LTC3526B will maintain voltage regulation
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 LTC3526/LTC3526B output disconnect feature allows
output short circuit while maintaining a maximum inter-
nally set current limit. 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_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 LTC3526/LTC3526B
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 LTC3526/LTC3526B can utilize small surface mount
chip inductors due to their fast 1MHz switching frequency.
Inductor values between 3.3µH and 6.8µH 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 cur-
rent. Increasing the inductance above 10µH will increase
size while providing little improvement in output current
capability.
The minimum inductance value is given by:
( ) _L
>
V_IN(MIN) • V_OUT(MAX) – V_IN(MIN)
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
GND 2
LTC3526
6 V_OUT
5 FB
MINIMIZE
TRACE ON FB
AND SW
+
V_IN
V_IN 3
4 SHDN
MULTIPLE VIAS
TO GROUND PLANE
3526 F02
Figure 2. Recommended Component Placement for Single Layer Board
3526bfd