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參數(shù)資料
| 型號(hào)： | LTC1505 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Constant-Current/Voltage High Efficiency Battery Charger(恒定電流/電壓,高效率電池電荷控制器) |
| 中文描述： | Constant-Current/Voltage高效電池充電器（恒定電流/電壓，高效率電池電荷控制器） |
| 文件頁(yè)數(shù)： | 13/16頁(yè) |
| 文件大?。?/td> | 197K |
| 代理商： | LTC1505 |
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13
LT1505
With divider current set at 15
μ
A, R4 = 2.465/15
μ
A = 162k
and,
R3
R4 V
2.465
2.465
162k 8.4
2.465
2.465
390k
BAT
=
(
)
=
)
=
Li-Ion batteries typically require float voltage accuracy of
1% to 2%. Accuracy of the LT1505 V
FB
voltage is
±
0.5%
at 25
°
C and
±
1% over the full temperature range. This
leads to the possibility that very accurate (0.1%) resistors
might be needed for R3 and R4. Actually, the temperature
of the LT1505 will rarely exceed 50
°
C in float mode
because charging currents have tapered off to a low level,
so 0.25% resistors will normally provide the required level
of overall accuracy.
APPLICATIO
S I
FOR
ATIO
U
W
U
U
V
BAT
1505 F04
8.4V
R3
390k
0.25%
R4
162k
0.25%
+
V
FB
LT1505
Figure 4. External Resistor Divider
Lithium-Ion Charging Completion
Some battery manufacturers recommend termination of
constant-voltage float mode after charge current has
dropped below a specified level (typically around 20% of
the full current) and a further time-out period of 30
minutes to 90 minutes has elapsed. Check with manufac-
turers for details. The LT1505 provides a signal at the
FLAG pin when charging is in voltage mode and current is
reduced to 20% of full current, assuming full charge
current is programmed to have 100mV across the current
sense resistor (V
RS1
). The comparator E6 in the Block
Diagram compares the charge current sample I
PROG
to the
output current I
VA
voltage amplifier VA. When the charge
current drops to 20% of full current, I
PROG
will be equal to
0.25 I
VA
and the open-collector output V
FLAG
will go low
and can be used to start an external timer. When this
feature is used, a capacitor of at least 0.1
μ
F is required at
the CAP pin to filter out the switching noise and a pull-up
resistor is also needed at the FLAG pin. If this feature is not
used, C6 is not needed.
Very Low Dropout Operation
The LT1505 can charge the battery even when V
CC
goes
as low as 0.5V above the combined voltages of the
battery and the drops on the sense resistor as well as
parasitic wiring. This low V
CC
sometimes requires a duty
factor greater then 99% and TGATE stays on for many
switching cycles. While TGATE stays on, the voltage
V
BOOST
across the capacitor C2 drops down because
TGATE control circuits require 2mA DC current. C2 needs
to be recharged before V
BOOST
drops too low to keep the
topside switch on. A unique design allows the LT1505 to
operate under these conditions; the comparator A2 moni-
tors V
BOOST
and when it drops from 9.1V to 6.9V, TGATE
will be turned off for about 0.2
μ
s to recharge C2. Note that
the LT1505 gets started the same way when power turns
on and there is no initial V
BOOST
.
It is important to use 0.56
μ
F or greater value for C2 to hold
V
BOOST
up for a sufficient amount of time.
When minimum operating V
CC
is less than 2.5V above the
battery voltage, D3 and C4 (see Figure 1) are also needed
to bootstrap V
BOOSTC
higher than V
CC
to bias the current
amplifier CA1. They are not needed if V
CC
is at least 2.5V
higher than V
BAT
. The PFET M3 is optional and can be
replaced with a diode if V
IN
is at least 3V higher than V
BAT
.
The gate control pin INFET turns on M3 when V
IN
gets up
above the undervoltage lockout level set by R5 and R6 and
is clamped internally to 8V below V
CC
. In sleep mode when
V
IN
is removed, INFET will clamp M3 V
SG
to 0.2V.
Shutdown
When adapter power is removed, V
CC
will drift down and
be held by the body diode of the topside NFET switch. As
soon as V
CC
goes down to 0.2V above V
BAT
, the LT1505 will
go into sleep mode drawing only 10
μ
A from the battery.
There are two ways to stop switching: pulling the SHDN
pin low or pulling the V
C
pin low. Pulling the SHDN pin low
will also turn off V
GBIAS
and CA1 input currents. Pulling the
V
C
pin low will only stop switching and V
GBIAS
stays high.
Make sure there is a pull-up resistor on the SHDN pin even
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