参数资料
型号: LT3651IUHE-4.2#TRPBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: BATTERY CHARGE CONTROLLER, 1100 kHz SWITCHING FREQ-MAX, PQCC36
封装: 5 X 6 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, QFN-36
文件页数: 5/22页
文件大小: 213K
代理商: LT3651IUHE-4.2#TRPBF
LT3651-4.1/LT3651-4.2
13
365142fb
BAT Output Decoupling
It is recommended that the LT3651 charger output have
a decoupling capacitor. If the battery can be disconnected
from the charger output this capacitor is required. The
value of this capacitor (CBAT) is related to the minimum
operational VIN voltage such that:
CBAT ≈ 20μF +
350μF
VIN(MIN)
The voltage rating on CBAT must meet or exceed the bat-
tery float voltage.
RSENSE: Charge Current Programming
The LT3651 charger is configurable to charge at average
currents as high as 4A (see Figure 1). If RNG/SS maximum
voltage is not limited, the inductor sense resistor, RSENSE,
has 95mV across it at maximum charge current so:
RSENSE =
0.095V
IMAX(AVG)
where IMAX(AVG) is the maximum average charge current.
RSENSE is 24mΩ for a 4A charger.
Inductor Selection
The primary criteria for inductor value selection in the
LT3651 charger is the ripple current created during switch-
ing. Ripple current, ΔIMAX, is typically set within a range of
25% to 35% of the maximum charge current, IMAX. This
percentage typically gives a good compromise between
losses due to ripple and inductor size. An approximate
formula for inductance is:
L
=
VBAT + VF
ΔIMAX fOSC MHz
()
1–
VBAT + VF
VIN + VF
μH
()
Worse-case ripple is at high VIN and high VBAT. VF is the
forward voltage of the synchronous switch (approximately
0.14V at 4A). Figure 2 shows inductance for the case of a
4A charger. The inductor must have a saturation current
equal to or exceeding the maximum peak current in the
inductor. Peak current is IMAX + ΔIMAX/2.
Magnetics vendors typically specify inductors with maxi-
mum RMS and saturation current ratings. Select an induc-
tor that has a saturation current rating at or above peak
current, and an RMS rating above IMAX. Inductors must
also meet a maximum volt-second product requirement.
If this specification is not in the data sheet of an inductor,
consult the vendor to make sure the maximum volt-second
product is not being exceeded by your design. The minimum
required volt-second product is approximately:
V
BAT
f
OSC(MHz)
1–
V
BAT
V
IN(MAX)
Vs
()
APPLICATIONS INFORMATION
Figure 2. Inductance (L) vs Maximum VIN
Figure 1. Programming Maximum Charge Current Using RSENSE
VIN(MAX) (V)
5
L
(μH)
2
3
25
365142 F02
1
0
10
15
20
30
4
IMAX = 4A
fOSC = 1MHz
25% TO 35% RIPPLE
SW
BOOST
SENSE
RSENSE
LT3651
BAT
365142 F01
+
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LT3651IUHE-4.1#PBF BATTERY CHARGE CONTROLLER, 1100 kHz SWITCHING FREQ-MAX, PQCC36
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