LTC4015_15 Datasheet, PDF (44/76 Pages) Linear Technology – Multichemistry Buck Battery Charger Controller with Digital Telemetry System

LTC4015
Applications Information
If testing is to be done with a electronic load in constant-
current mode, care must be taken if using CC mode that
instability may occur. A few milli Farads on V_BAT, with
enough series R (e.g. ESR) to give a zero around 1kHz
range can help in this situation.
Inductor Selection
The operating frequency and inductor selection are interre-
lated. Higher operating frequencies allow the use of smaller
inductor and capacitor values but generally also results in
lower efficiency because of MOSFET switching and gate
charge losses. In addition, the effect of inductor value
on ripple current must also be considered. The inductor
ripple current decreases with higher inductance or higher
frequency and increases with higher V_IN. Accepting larger
values of ripple current allows the use of low inductances,
but results in higher output voltage ripple and greater core
losses. For the LTC4015, the best overall performance will
be attained if the inductor is chosen to be:
L = ^{VBAT •(1– VBAT / VIN(MAX))}
0.25 • f •I_CHG(MAX)
Where V_BAT is the highest BATSENS voltage, V_IN(MAX) is
the maximum input voltage, I_CHG(MAX) is the maximum
regulated charge current and f_SW is the switching fre-
quency. Using these equations, the inductor ripple will be
at most 25% of I_CHG(MAX). Once the value for L is known,
the type of inductor core must be selected. Ferrite cores
are recommended for their very low core loss. Selection
criteria should concentrate on minimizing copper loss and
preventing saturation. Ferrite core material saturates hard,
which means that inductance collapses abruptly when the
peak design current is exceeded. This causes an abrupt
increase in inductor ripple current and consequent out-
put voltage ripple. Do not allow the core to saturate! The
saturation current for the inductor should be at least 60%
higher than the maximum regulated current, I_CHG(MAX).
C_SYS and C_BAT Capacitance
The specification for C_SYS will be determined by the desired
ripple voltage:
∆V_SYS
=
V_BAT
V_SYS
_1–

V_BAT
V_SYS
CI^CSHYGS(•^Mf^A_S^X_W^{) +ICHG(MAX) • ESRCSYS}
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V_BAT/V_SYS). To prevent
large voltage transients, a low ESR capacitor sized for
the maximum RMS current must be used. The maximum
RMS capacitor current is given by:
I_RMS
=
I_CHG(MAX)
V_BAT
V_SYS
V_{BAT – 1}
V_SYS
This formula has a maximum at V_SYS = 2 V_BAT, where
I_RMS = I_CHG(MAX)/2. This simple worst-case condition
is commonly used for design because even significant
deviations do not offer much relief. Medium voltage (20V
to 35V) ceramic, tantalum, OS-CON and switcher rated
electrolytic capacitors can be used as input capacitors.
Sanyo OS-CON SVP, SVPD series; Sanyo POSCAP TQC
series or aluminum electrolytic capacitors from Panasonic
WA series or Cornel Dublilier SPV series, in parallel with
a couple of high performance ceramic capacitors, can be
used as an effective means of achieving low ESR and high
bulk capacitance.
The purpose of the V_BAT capacitor is to filter the induc-
tor current ripple as well as to stabilize the charger if the
battery is not present or has high BSR. The V_BAT ripple
(ΔV_BAT) is approximated by:
ΔV_BAT
=
ΔI_P-P


8
•
1
C_BAT
•
f_SW
+ESR_CBAT^{}
Where f_SW is the operating frequency, C_BAT is the ca-
pacitance on V_BAT and ΔI_P-P is the ripple current in the
inductor. The output ripple is highest at maximum input
voltage since ΔI_P-P increases with input voltage.
4015f
44
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