MP2619 – 2A, 24V INPUT, 600kHz 2-3CELL SWITCHING LI-ION BATTERY CHARGER
MP2619 Rev. 1.01 www.MonolithicPower.com 19
5/28/2015 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
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Selecting the Inductor
A 1µH to 10µH inductor is recommended for
most applications. The inductance value can be
derived from the following equation.
OUT IN OUT
IN L OSC
V(VV)
L
VIf
(10)
Where I
L
is the inductor ripple current. V
OUT
is
the 2/3 cell battery voltage.
Choose inductor current to be approximately
30% if the maximum charge current, 2A. The
maximum inductor peak current is:
L
L(MAX) CHG
I
II
2
(11)
Under light load conditions below 100mA, larger
inductance is recommended for improved
efficiency.
For optimized efficiency, the inductor DC
resistance is recommended to be less than
200m.
NTC Function
As Figure 8 shows, the low temperature
threshold and high temperature threshold are
preset internally via a resistive divider, which are
73%·VREF33 and 30%·VREF33. For a given
NTC thermistor, we can select appropriate R3
and R6 to set the NTC window.
In detail, for the thermistor (NCP18XH103) noted
in above electrical characteristic,
At 0ºC, R
NTC_Cold
= 27.445k;
At 50ºC, R
NTC_Hot
= 4.1601k.
Assume that the NTC window is between 0ºC
and 50ºC, the following equations could be
derived:
73%
VREF33
V
R6//RR3
R6//R
TH_Low
NTC_Cold
NTC_Cold
(12)
30%
VREF33
V
R6//RR3
R6//R
TH_High
NTC_Hot
NTC_Hot
(13)
According to equation (12) and equation (13), we
can find that R3 = 9.63k and R6 = 505k.
To be simple in project, making R3=10k and R6
no connect will approximately meet the
specification.
Figure 8— NTC function block
Selecting the Input Capacitor
The input capacitor reduces the surge current
drawn from the input and also the switching noise
from the device. The input capacitor impedance
at the switching frequency should be less than
the input source impedance to prevent high
frequency switching current passing to the input.
Ceramic capacitors with X5R or X7R dielectrics
are highly recommended because of their low
ESR and small temperature coefficients. For
most applications, a 4.7µF capacitor is sufficient.
Selecting the Output Capacitor
The output capacitor keeps output voltage ripple
small and ensures regulation loop stability. The
output capacitor impedance should be low at the
switching frequency. Ceramic capacitors with
X5R or X7R dielectrics are recommended.
PC Board Layout
The high frequency and high current paths (GND,
IN and SW) should be placed to the device with
short, direct and wide traces. The input capacitor
needs to be as close as possible to the IN and
GND pins. The external feedback resistors
should be placed next to the FB pin. Keep the
switching node SW short and away from the
feedback network.
