參數(shù)資料
型號: LTC4008EGN-1#TRPBF
廠商: LINEAR TECHNOLOGY CORP
元件分類: 穩(wěn)壓器
英文描述: 4A, High Efficiency, Multi-Chemistry Battery Charger; Package: SSOP; No of Pins: 20; Temperature Range: -40°C to +125°C
中文描述: BATTERY CHARGE CONTROLLER, 345 kHz SWITCHING FREQ-MAX, PDSO20
封裝: 0.150 INCH, LEAD FREE, PLASTIC, SSOP-20
文件頁數(shù): 5/24頁
文件大?。?/td> 221K
代理商: LTC4008EGN-1#TRPBF
LTC4008
13
4008fb
APPLICATIONS INFORMATION
Charger Current Programming
The basic formula for charging current is:
I
Vk
R
V
R
CHARGE MAX
REF
PROG
SENSE
()
.
/
– .
=
Ω
3 01
0 035
VREF = 1.19V. This leaves two degrees of freedom: RSENSE
and RPROG. The 3.01k input resistors must not be altered
since internal currents and voltages are trimmed for this
value. Pick RSENSE by setting the average voltage between
CSP and BAT to be close to 100mV during maximum
charger current. Then RPROG can be determined by solving
the above equation for RPROG.
R
Vk
RI
V
PROG
REF
SENSE
CHARGE MAX
=
Ω
+
.
()
301
0 035
Table 2. Recommended RSNS and RPROG Resistor Values
IMAX (A)
RSENSE (Ω) 1%
RSENSE (W)
RPROG (kΩ) 1%
1.0
0.100
0.25
26.7
2.0
0.050
0.25
26.7
3.0
0.033
0.5
26.7
4.0
0.025
0.5
26.7
Charging current can be programmed by pulse width
modulating RPROG withaswitchQ1toRPROG atafrequency
higher than a few kHz (Figure 5). CPROG must be increased
to reduce the ripple caused by the RPROG switching. The
compensation capacitor at ITH will probably need to be
increased also to improve stability and prevent large
overshoot currents during start-up conditions. Charging
current will be proportional to the duty cycle of the switch
with full current at 100% duty cycle and zero current when
Q1 is off.
Maintaining C/10 Accuracy
The C/10 comparator threshold that drives the FLAG pin
has a xed threshold of approximately VPROG = 400mV.
This threshold works well when RPROG is 26.7k, but will
not yield a 10% charging current indication if RPROG is
a different value. There are situations where a standard
value of RSENSE will not allow the desired value of charging
current when using the preferred RPROG value. In these
cases, where the full-scale voltage across RSENSE is within
±20mV of the 100mV full-scale target, the input resistors
connected to CSP and BAT can be adjusted to provide the
desired maximum programming current as well as the
correct FLAG trip point.
For example, the desired max charging current is 2.5A but
the best RSENSE value is 0.033Ω. In this case, the volt-
age across RSENSE at maximum charging current is only
82.5mV, normally RPROG would be 30.1k but the nominal
FLAG trip point is only 5% of maximum charging current.
If the input resistors are reduced by the same amount as
the full-scale voltage is reduced then, R4 = R5 = 2.49k
and RPROG = 26.7k, the maximum charging current is
still 2.5A but the FLAG trip point is maintained at 10%
of full scale.
There are other effects to consider. The voltage across the
current comparator is scaled to obtain the same values as
the 100mV sense voltage target, but the input referred sense
voltage is reduced, causing some careful consideration of
the ripple current. Input referred maximum comparator
threshold is 117mV, which is the same ratio of 1.4x the
DC target. Input referred IREV threshold is scaled back to
–24mV. The current at which the switcher starts will be
reduced as well so there is some risk of boost activity.
These concerns can be addressed by using a slightly larger
inductor to compensate for the reduction of tolerance to
ripple current.
RZ
102k
CPROG
4008 F05
LTC4008
PROG
Q1
2N7002
RPROG
0V
5V
10
Figure 5. PWM Current Programming
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