Because the V
PP-C
and V
PP-ESR
are out of phase, the RMS
value can be used to get an approximate value of the peakto-peak ripple:
30000138
Note that the output voltage ripple is dependent on the inductor current ripple and the equivalent series resistance of the
output capacitor (R
ESR
). The R
ESR
is frequency dependent as
well as temperature dependent. The R
ESR
should be calculated with the applicable switching frequency and ambient
temperature.
Capacitor
Min
Value
Unit Description Recommended Type
C
VDD
4.7
μF
Charger Input Capacitor Ceramic, 6.3V, X5R
C
CHG_DET
4.7
μF
Charger Input Capacitor Ceramic, 6.3V, X5R
C
USB
4.7
μF
USB Power (V
BUS
) Capacitor Ceramic, 6.3V, X5R
C
BATT
4.7
μF
Li-ion Battery Capacitor Ceramic, 6.3V, X5R
C
LDO1
1.0
μF
LDO Output Capacitor Ceramic, 6.3V, X5R
C
LDO2
1.0
μF
LDO Output Capacitor Ceramic, 6.3V, X5R
C
VREFH
0.1
μF
Bypass Capacitor for Internal Voltage Reference Ceramic, PolyPropylene and
Polycarbonate Film
C
VIN2,3
10
μF
Buck1, Buck2 Input Capacitor Ceramic, 6.3V, X5R
CVBUCK1,2 10
μF
BUCK1,2 Output Capacitor Ceramic, 6.3V, X5R
C
SW3
22
μF
BUCK3 Output Capacitor Ceramic, 6.3V, X5R
C
VIN1
1
μF
LDO Bypass Capacitor Ceramic, 6.3V, X5R
C
VIN4
10
μF
Buck Bypass Capacitor Ceramic, 6.3V, X5R
Schottky Diode on Charger Input CHG_IN
A Schottky diode is required in the external adapter path to
block the reverse current from either the USB or the battery
source. The most critical parameter in the selection of the right
Schottky diode is the leakage current, which needs to be below 10 µA over the temperature range in order to prevent false
detection of the presence of an external adapter. In addition
the Schottky diode should have a maximum voltage rating of
10V or higher. The current rating depends on the current limit
of the adapter. The forward voltage should be limited to 500
mV at its maximum current. The recommended Schottky
diode is MBRA210ET3 from ON Semiconductor which has a
reverse leakage current under 1 µA at room temperature and
a forward voltage drop of 500 mV at their max rated current
IF = 2A.
RESISTORS
Battery Thermistor
The LP3913 battery thermistor bias provided by the TS pin is
tailored to thermistors with the following specification:
Negative Temperature Coefficient
100 kΩ resistance
A suitable solution is available from AVX thermistors:
AVXNB21250104
http://www.avxcorp.com/docs/Catalogs/nb21-23.pdf
I2C Pullup Resistors
I2C_SDA, I2C_SCL terminals need to have pullup resistors
connected to the V
DDIO
pin. V
DDIO
must be connected to a
power supply that is less than or equal to VDD, such as Buck2.
The values of the pull-up resistors (typ. ≈1.8 kΩ) are determined by the capacitance of the bus. Too large of a resistor
combined with a given bus capacitance will result in a rise
time that would violate the max rise time specification. Too
small of a resistor will result in a contention with the pull-down
transistor on either slave(s) or master.
R
IREF
Resistor
The current through this resistor is used as a reference current that biases many analog circuits inside the LP3913 and
needs to have a resistance of 121 kΩ ±1%
R
ISENSE
Resistor
The current through this resistor is used as a reference current for the charge current. The accuracy of the ADC is
dependent on the tolerance of this resistor. R
ISENSE
needs to
have a resistance of 4.64 kΩ ±1% tolerance.
Operation without I2C Interface
Operation of the LP3913 without the I2C interface is possible
if the system can operate with default values for the DC/DC
converters and the charger. (Read below: Factory programmable options). The I2C-less system must use the POWERACK pin to power cycle the LP3913.
I2C Master Power Concern
The processor that contains the I2C master should be powered by BUCK1 or LDO2 as these converters require no I2C
access to enable/disable them. If the I2C master were to be
powered by a DC/DC converter that is enable/disabled
through a control register, then a corrupted application software execution could by accident disable the power to the
I2C master, which in this case has no means to recover. It is
possible that the regulator connected to V
DDIO
could accidentally disable, in which case the processor should be able to
recognize that communication has been broken and then
power down the system to allow for a clean restart.
System Operation When the Load Current Exceeds the
USB or Adapter Current Limit
In the event that the system requires current that exceeds the
current limit of either the USB or the adapter source, then the
battery can provide the extra power provided that it has been
charged. It is clear that a long sustained overload will eventually discharge the battery such that its extra power will no
55 www.national.com
LP3913