MP2623 – 2A, 24V INPUT, 1.1MHz 1- OR 2-CELL SWITCHING LIFEPO
4
BATTERY CHARGER
MP2623 Rev. 1.0 www.MonolithicPower.com 15
12/26/2013 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2013 MPS. All Rights Reserved.
APPLICATION INFORMATION
Setting the Charge Current
RS1 sets the MP2623 charge current (See
Typical Application). Determine the current with
the following equation:
mRS1
200mV
AI
CHG
(1)
Selecting the Inductor
Use a 1µH-to-10µH inductor for most applications.
Calculate the inductance value from the following
equation.
OUT IN OUT
IN L OSC
V(VV)
L
VIf
(2)
Where I
L
is the inductor ripple current. Choose
I
L
to be approximately 30% of the maximum
charge current, 2A. V
OUT
is the 1- or 2-cell battery
voltage.
The maximum inductor peak current is:
L
L(MAX) CHG
I
II
2
(3)
Under light-load conditions (below 100mA), use a
larger inductor value to improve efficiency.
Select an inductor with a DC resistance of less
than 200m to optimize efficiency.
NTC Function
Figure 6 shows that the low temperature
threshold and high-temperature threshold are
preset internally to 73.5%·VREF33 and
29.5%·VREF33, respectively, using a resistor
divider. For a given NTC thermistor, we can
select appropriate R3 and R6 resistors to set the
NTC window.
For the thermistor (NCP18XH103) noted in the
electrical characteristic previous,
At 0°C, R
NTC_Cold
= 27.445k;
At 50°C, R
NTC_Hot
= 4.1601k.
Assuming that the NTC window is between 0°C
and 50°C, we can derive the following equations:
NTC _ Cold TH _ Low
NTC _ Cold
R6 //R V
73.5%
R3 R6 // R VREF33
(4)
TH _ High
NTC _ Hot
NTC _ Hot
V
R6 //R
29.5%
R3 R6 // R VREF33
(5)
According to equation (4) and equation (5), R3 =
9.63k and R6 = 505k.
Simplifying, select R3=10k and R6 no connect to
approximate the estimate.
NTC
VREF33
Low Temp Threshold
High Temp Threshold
R
NTC
R3
R6
V
TH_Low
V
TH_High
Figure 6: NTC function block
Selecting the Input Capacitor
The input capacitor reduces the surge current
drawn from the input and the switching noise
from the device. Chose an input capacitor with an
impedance at the switching frequency less than
the input source impedance to prevent a high-
frequency switching current. Use ceramic
capacitors with X5R or X7R dielectrics with low
ESR and small temperature coefficients. A 4.7µF
capacitor is sufficient for most applications.
Selecting the Output Capacitor
The output capacitor limits output voltage ripple
and ensures regulator-loop stability. The output
capacitor impedance should be low at the
switching frequency. Use ceramic capacitors with
X5R or X7R dielectrics.
PC Board Layout
Connect the high frequency and high current
paths (GND, IN, and SW) to the device with short,
wide, and direct traces. Place the input capacitor
as close as possible to the IN and GND pins.
Place the external feedback resistors next to the
FB pin. Keep the switching node SW short and
away from the feedback network.
