LTC3548-1 Datasheet, PDF (12/16 Pages) Linear Technology – Dual Synchronous, Fixed Output 2.25MHz Step-Down DC/DC Regulator

LTC3548-1
APPLICATIONS INFORMATION
First, calculate the inductor value for about 30% ripple
current at maximum V_IN:
L
=
1.8V
2.25MHz • 240mA
•



1–
1.8V
4.2V


=
1.9μH
Choosing a vendor’s closest inductor value of 2.2μH,
results in a maximum ripple current of:
I_L
=
1.8V
2.25MHz •
2.2μ
•


1

1.8V
4.2V


=
207mA
For cost reasons, a ceramic capacitor will be used. C_OUT
selection is then based on load step droop instead of ESR
requirements. For a 5% output droop:
C_OUT
≈
1.8
800mA
2.25MHz • (5%
•
1.8V)
=
7.1μF
A good standard value is 10μF. Since the output impedance
of a Li-lon battery is very low, C_IN is typically 10μF.
Figure 3 shows the complete schematic for this design
example.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3548-1. These items are also illustrated graphically
in the layout diagram of Figure 4. Check the following in
your layout:
1. Does the capacitor C_IN connect to the power V_IN (Pin 3)
and GND (exposed pad) as close as possible? This
capacitor provides the AC current to the internal power
MOSFETs and their drivers.
2. The feedback lines from V_OUT should be routed away
from noisy traces such as the SW line and its trace
should be minimized.
3. Are the C_OUT and L1 closely connected? The (–) plate of
C_OUT returns current to GND and the (–) plate of C_IN.
4. Keep sensitive components away from the SW pins.
The input capacitor C_IN should be routed away from
the SW traces and the inductors.
5. A ground plane is preferred, but if not available, keep the
signal and power grounds segregated with small signal
components returning to the GND pin at one point and
should not share the high current path of C_IN or C_OUT.
6. Flood all unused areas on all layers with copper. Flooding
with copper will reduce the temperature rise of power
components. These copper areas should be connected
to V_IN or GND.
V_IN
2.7V TO 5.5V
V_OUT2
1.575V
400mA
C_OUT2
10μF
CER
C_IN
10μF
CER
4.7μH
C_FF2
330pF
V_IN RUN1 RUN2
LTC3548-1
SW2
SW1
V_OUT2
V_OUT1
V_FB2
GND
V_FB1
2.2μH
C_FF1
330pF
V_OUT1
1.8V
800mA
C_OUT1
10μF
CER
3548-1 F01
Figure 3. LTC3548-1 Typical Application
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