LTC3535 Datasheet, PDF (12/16 Pages) Linear Technology – Dual Channel 550mA 1MHz Synchronous Step-Up DC/DC Converter

LTC3535
APPLICATIONS INFORMATION
V_IN > V_OUT OPERATION
The LTC3535 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 LTC3535 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 per channel).
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 LTC3535 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.
SHDN
V_OUT1
V_IN1
GND
GND
COMPONENT SELECTION
Inductor Selection
The LTC3535 can utilize small surface mount chip induc-
tors 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 current.
Increasing the inductance above 10μH will increase com-
ponent size while providing little improvement in output
current capability.
The minimum inductance value is given by:
( ) L > VIN(MIN) • VOUT(MAX) – VIN(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 dependent
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 inductor
current without saturating. Molded chokes and some chip
inductors usually do not have enough core area to support
the peak inductor current of 750mA seen on the LTC3535.
To minimize radiated noise, use a shielded inductor. See
Table 1 for suggested components and suppliers.
V_OUT2
V_IN2
SHDN
Figure 1. Recommended Component Placement
3535f
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