LTC3552 Datasheet, PDF (20/24 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger and Dual Synchronous Buck Converter

LTC3552
APPLICATIO S I FOR ATIO
Reverse Polarity Input Voltage Protection
In some applications, protection from reverse polarity
voltage on V_IN is desired. If the supply voltage is high
enough, a series blocking diode can be used. In other cases,
where the voltage drop must be kept low, a P-regulator
MOSFET can be used (as shown in Figure 4).
DRAIN-BULK
DIODE OF FET
LTC3552
V_IN
V_IN
3552 F04
Figure 4. Low Loss Input Reverse
Polarity Protection
Design Example
As a design example, assume the LTC3552 is used in
a single lithium-ion battery-powered cellular phone ap-
plication.
Starting with the charger, choosing R_PROG to be 1.24k
programs the charger for 806mA. A good rule of thumb
for I_TERMINATE is one tenth the full charge current, so R_ITERM
is picked to be 1.24k (I_TERMINATE = 80mA).
For the switching regulator, V_CC will be operating from a
maximum of 4.2V down to about 2.7V. The load requires
a maximum of 800mA in active mode and 2mA in standby
mode. Regulator 1 output voltage is 1.8V. Since the load
still needs power in standby, Burst Mode operation is se-
lected for good low load efficiency (connect MODE/SYNC
to V_CC). First, calculate the inductor value for about 30%
ripple current at maximum V_CC:
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 • 2.2µH
⎛⎝⎜_1−
1.8V ⎞
4.2V ⎠⎟
=
208mA
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
=
2.5
800mA
2.25MHz • (5%
•
2.5V)
=
7.1µF
A good standard value is 10µF. Since the impedance of a
Li-Ion battery is very low, C_IN is typically 10µF. The output
voltage can now be programmed by choosing the values
of R1 and R2. To maintain high efficiency, the current in
these resistors should be kept small. Choosing 2µA with
0.6V feedback voltage makes R1 ~300k. A close standard
1% resistor is 301k, and R2 is then 604k. The ^⎯P^⎯O^⎯R pin
is an open-drain output and requires a pull-up resistor. A
100k resistor is used for adequate speed. Figure 2 shows
the complete schematic for this design example.
3552f
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