LTC3528_15 Datasheet, PDF (11/18 Pages) Linear Technology – 1A, 1MHz Synchronous Step-Up DC/DC Converters

LTC3528/LTC3528B
APPLICATIONS INFORMATION
V_IN > V_OUT OPERATION
The LTC3528/LTC3528B 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 LTC3528/LTC3528B output disconnect feature allows
an 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 750mA (typical).
SCHOTTKY DIODE
Although 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 LTC3528/LTC3528B de-
mands careful attention to board layout. A careless layout
will not produce the advertised performance. Figure 2
shows the recommended component placement. A large
ground copper area with the package backside metal pad
properly soldered will help to lower the chip temperature.
A multilayer board with a separate ground plane is ideal,
but not absolutely necessary.
COMPONENT SELECTION
Inductor Selection
The LTC3528/LTC3528B can utilize small surface mount
chip inductors due to their fast 1MHz switching frequency.
Inductor values between 2.2µH 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 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)
1.2 • Ripple • V_OUT(MAX)
µH
where:
Ripple = Allowable inductor current ripple (amps peak-
to-peak)
V_IN(MIN) = Minimum input voltage
V_OUT(MAX) = Maximum output voltage
+
V_IN
C_IN
SHDN 1
8 V_IN
FB 2
PGOOD 3
LTC3528
7 SGND
6 PGND
V_OUT 4
C_OUT
5 SW
3528 F01
MULTIPLE VIAS
TO GROUND PLANE
Figure 2. Recommended Component Placement for Single Layer Board
3528fd
11