LT1571-1 LINER [Linear Technology], LT1571-1 Datasheet - Page 11

• Manufacturer Part Number: LT1571-1
• Description: Constant-Current/ Constant-Voltage Battery Charger with Preset Voltage and Termination Flag
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
lower value and maintained as a constant trickle charge.
An intermediate “top off” current may also be used for a
fixed time period to reduce total charge time.
NiMH batteries are similar in chemistry to NiCd but have
two differences related to charging. First, the inflection
characteristic in battery voltage as full charge is
approached is not nearly as pronounced. This makes it
more difficult to use – dV/dt as an indicator of full charge,
and an increase in temperature is more often used with a
temperature sensor located in the battery pack. Secondly,
constant trickle charge may not be recommended. In-
stead, a moderate level of current is used on a pulse basis
( 1% to 5% duty cycle) with the time-averaged value
substituting for a constant low trickle.
Thermal Calculations
If the LT1571 is used for charge currents above 0.4A, a
thermal calculation should be done to ensure that junction
temperature will not exceed 125 C. Power dissipation in
the IC is caused by bias and driver current, switch resis-
tance, switch transition losses and the current sense
resistor. The following equations show that maximum
practical charge current for the 16-pin SSOP package
(75 C/W thermal resistance) is about 1.2A for an 8.4V
battery and 1.4A for a 4.2V battery. This assumes a 60 C
maximum ambient temperature. The 28-pin SSOP, with a
thermal resistance of 40 C/W, can provide a full 1.5A
charge current in many situations.
** COILTRONICS CTX33-2
* TOKIN OR MARCON CERAMIC
L1**
33 H
SURFACE MOUNT
0.22 F
C1
D2
1N914
Figure 7. Charging NiMH or NiCd Batteries with
Constant Current (Efficiency at 0.5A 90%)
1N5819
D1
SW
BOOST
GND
SENSE
LT1571-1
U
PROG
BAT
V
CC
V
C
U
I
BAT
+
0.1 F
1 F
C
10 F
C
22 F
TANT
IN
OUT
300
*
1k
W
+
1N5819
D3
100k
2V TO
20V
R1
V
(WALL ADAPTER)
ON: I
OFF: I
R2
11k
IN
U
BAT
BAT
1571 F07
Q1
VN2222
= 0.5A
= 0.05A
R
t
f = 200kHz (500kHz for LT1571-5)
Example: V
Total power in the IC is:
Temperature rise will be (0.88W)(40 C/W) = 35 C. This
assumes that the LT1571 is properly heat sunk by con-
necting all fused ground pins to the expanded traces and
that the PC board has a backside or internal plane for heat
spreading.
The P
diode D2 to a lower system voltage (lower than V
OL
SW
0.17 + 0.13 + 0.32+ 0.26 = 0.88W
P
P
P
P
P
P
P
P
= Effective switch overlap time 10ns
SENSE
SENSE
BIAS
DRIVER
SW
BIAS
DRIVER
SW
= Switch ON resistance 0.35
DRIVER
I
I
IN
V
BAT
V
BAT
3 5
3 5
term can be reduced by connecting the boost
BAT
BAT
V
V
.
.
0
= 15V, V
0
IN
IN
I
I
. . 18
. . 18
mA V
mA V
BAT
BAT
2
2
2
2
R
R
V
V
SW
SW
7 5
IN
7 5
IN
V
V
.
.
IN
IN
I
I
BAT
BAT
BAT
BAT
BAT
55
55
mA
mA
V
V
BAT
= 8.4V, I
BAT
V
V
2
2
1 5
1 5
2
2
IN
IN
.
.
1
1
mA V
0 012
mA V
0 012
.
.
V
V
LT1571 Series
30
30
BAT
BAT
t
t
BAT
OL
OL
BAT
BAT
I
I
= 1.2A;
BAT
BAT
V
V
IN
IN
I
I
BAT
BAT
11
f
f
BAT
)