ltc4007eufd-1-trpbf Linear Technology Corporation, ltc4007eufd-1-trpbf Datasheet - Page 13

• Manufacturer Part Number: ltc4007eufd-1-trpbf
• Description: 4a, High Efficiency, Standalone Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Battery Detection
It is generally not good practice to connect a battery while
the charger is running. The timer is in an unknown state
and the charger could provide a large surge current into
the battery for a brief time. The Figure 5 circuit keeps the
charger shut down and the timer reset while a battery is not
connected.
Charger Current Programming
The basic formula for charging current is:
This leaves two degrees of freedom: R
The 3.01k input resistors must not be altered since internal
currents and voltages are trimmed for this value. Pick
R
BAT to be close to 100mV during maximum charger
current. Then R
above equation for R
Table 2. Recommended R
SENSE
I
MAX
V
I
R
CHARGE MAX
1.0
2.0
3.0
4.0
REF
PROG
(A)
= 1.19V
by setting the average voltage between C
=
(
R
R
SENSE
SENSE
SWITCH CLOSED
WHEN BATTERY
)
ADAPTER
PROG
0.100
0.050
0.033
0.025
=
POWER
CONNECTED
(Ω) 1%
U
V
REF
•
PROG
I
can be determined by solving the
V
CHARGE MAX
SNS
REF
Figure 5
• .
U
.
3 01
and R
• .
R
3 01
SENSE
(
k
PROG
0.25
0.25
Ω
0.5
0.5
R
24 SHDN
1 DCIN
k
SENSE
/
Ω
(W)
)
LTC4007
W
R
Resistor Values
+
PROG
0 035
4007 F05
SENSE
.
R
– .
PROG
V
0 035
and R
26.7
26.7
26.7
26.7
U
(kΩ) 1%
SP
V
PROG
and
.
Charging current can be programmed by pulse width
modulating R
quency higher than a few kHz (Figure 6). C
increased to reduce the ripple caused by the R
switching. The compensation capacitor at ITH will prob-
ably need to be increased also to improve stability and
prevent large overshoot currents during start-up condi-
tions. 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 fixed threshold of approximately V
This threshold works well when R
not yield a 10% charging current indication if R
different value. There are situations where a standard
value of R
current when using the preferred R
cases, where the full-scale voltage across R
±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 R
across R
82.5mV, normally R
FLAG trip point is only 5% of maximum charging current.
If the input resistors are reduced by the same amount as
SENSE
SENSE
SENSE
Figure 6. PWM Current Programming
5V
0V
PROG
will not allow the desired value of charging
value is 0.033Ω. In this case, the voltage
at maximum charging current is only
with a switch Q1 to R
PROG
R
PROG
LTC4007
would be 30.1k but the nominal
PROG
11
Q1
2N7002
4007 F06
PROG
C
PROG
PROG
LTC4007
is 26.7k, but will
R
102k
PROG
value. In these
Z
SENSE
PROG
PROG
= 400mV.
PROG
at a fre-
must be
is within
13
PROG
4007fa
is a