LTC3526 Linear Technology, LTC3526 Datasheet - Page 10

• Manufacturer Part Number: LTC3526
• Description: 500mA 1MHz Synchronous Step-Up DC/DC Converters
• Manufacturer: Linear Technology

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LTC3526/LTC3526B
V
The LTC3526/LTC3526B will maintain voltage regulation
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.
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
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 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.
10
APPLICATIONS INFORMATION
IN
> V
OUT
OUT
OPERATION
will improve effi ciency by about 2%. Note
Figure 1. Recommended Component Placement for Single Layer Board
V
IN
+
GND
SW
V
IN
1
2
3
LTC3526
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:
where:
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 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
Ripple = Allowable inductor current ripple (amps peak-
peak)
V
V
L >
3526 F01
MULTIPLE VIAS
TO GROUND PLANE
IN(MIN)
OUT(MAX)
6
5
4
V
FB
SHDN
OUT
V
IN(MIN)
= Minimum input voltage
Ripple • V
= Maximum output voltage
MINIMIZE
TRACE ON FB
AND SW
• V
(
OUT(MAX)
OUT(MAX)
– V
IN(MIN)
)
3526bfb