Datasheet

22
LTC3445
3445fa
APPLICATIO S I FOR ATIO
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DESIGN EXAMPLE
As a design example, assume the LTC3445 is used in a
single lithium-ion battery-powered Intel PXA270 micro-
processor application. The battery will be operating from
a maximum of 4.2V down to about 2.7V. Also, the battery
will be connected to all three power pins on the LTC3445.
The desired LDO outputs are 1.3V with a 23mA load and
1.1V with a 14mA load. Since both LDO’s are the same, we
will select LDO1 for the 1.3V output and LDO2 for the 1.1V
output. Using Equations 4 and 5, and choosing R2 and R4
to be 604k, the values for R1 and R2 are 705k and 503k
respectively. Also, selecting a 10µF output capacitor pro-
vides adequate stability and transient reponses.
The PXA270’s V
CC
BATT requirement can be readily handled
by the LTC3445’s PowerPath control circuits. By simply
connecting a coin cell battery to V
BACKUP
, the PowerPath
control circuits regulate V
CC
BATT within the PXA270’s
requirements.
The buck regulator’s maximum load requirement for this
application is 300mA. Although the default start-up volt-
age for the buck regulator is 1.35V, ripple current is
greatest when the output voltage is programmed to 0.85V.
For ripple currents of 200mA and the main battery at 4.2V,
the required inductor value is 2.2µH (Equation 1). For best
efficiency choose a 400mA or greater inductor with less
than 0.3Ω series resistance. Choosing a 10µF output
capacitor with an ESR of 0.25Ω will generate a ripple
voltage of 52mV (Equation 3). In most cases, a ceramic
capacitor’s ESR will be less than 0.25Ω further reducing
the output ripple (see Figure 11). Note that as V
CC1
decreases or V
OUT
increases, the ripple current and ripple
voltage will decrease. The input capacitor, C
IN
, will require
an RMS current rating of at least 0.150A ≅ I
LOAD(MAX)
/2 at
temperature (Equation 2).
Figure 11. Design Example
LDO2FB
LDO2
SDA
SCL
1.1V
3445 F11
604k
10µF
CER
503k
V
CC
_SRAM
LDO1FB
LDO1
FB
SW
1.3V
0.85V TO
1.55V
SUMIDA
CDRH3D16/HP2R2
2.2µH
604k
3k 3k
10µF
CER
10µF
CER
10µF
CER
705k
3V_TYP
V
TRACK
V
BACKUP
V
CC
V
CC
_PLL
V
CC
_CORE
V
CC
BATT
LTC3445
LTC3406
BUCK
PWR_EN
BATTFAULT
SYS_EN
PGOOD
RUN
V
CC1
V
CC1
V
CC2
ADD7
ADD6
I
2
C BUS
3V
V
CC
_BATT
V
CC
_IO
INTEL PXA270
2.5V TO 5.5V
LITHIUM ION
3V
COIN CELL
nV
CC
_FAULT
20k
V
CC
nBATT_FAULT
20k