LTC3527-1_15 Datasheet, PDF (13/22 Pages) Linear Technology – Dual 800mA/400mA, 1.2MHz/2.2MHz Synchronous Step-Up DC/DC Converters

LTC3527/LTC3527-1
APPLICATIONS INFORMATION
PCB Layout Guidelines
The high speed operation of the LTC3527/LTC3527-1
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 LTC3527/LTC3527-1 can utilize small surface mount
inductors due to their fast 1.2MHz/2.2MHz switching
frequencies. Inductor values between 3.3μH and 4.7μH
are suitable for most 1.2MHz applications. Inductor values
between 1.5μH and 2.2μH are suitable for most 2.2MHz
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 > ^{VIN(MIN) • (VOUT(MAX) – VIN(MIN))}
f • Ripple • V_OUT(MAX)
where:
Ripple = Allowable inductor current ripple (amps
peak-to-peak)
V_IN(MIN) = Minimum input voltage
V_OUT(MAX) = Maximum output voltage
f = Oscillator frequency (MHz)
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 currents of 900mA (500mA)
seen on the LTC3527/LTC3527-1. To minimize radiated
noise, use shielded inductors. See Table 1 for suggested
components and suppliers.
MODE SHDN1
V_IN
GND
SHDN2 FSEL
GND
GND
V_IN1
V_IN2
V_OUT1
GND
V_OUT2
35271 F01
Figure 1. Recommended Component Placement for a Dual-Layer Board
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