LT1571-1 LINER [Linear Technology], LT1571-1 Datasheet - Page 12

• 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
LT1571 Series
instead of V
(V
Then,
For example, V
The average I
Total board area becomes an important factor when the
area of the board drops below about 20 square inches. The
graph in Figure 9 shows thermal resistance vs board area
for 2-layer and 4-layer boards. Note that 4-layer boards
have significantly lower thermal resistance, but both types
show a rapid increase for reduced board areas. Figure 10
shows actual measured lead temperature for chargers
operating at full current. Battery voltage and input voltage
will affect device power dissipation, so the data sheet
power calculations must be used to extrapolate these
readings to other situations.
Vias should be used to connect board layers together.
Planes under the charger area can be cut away from the
rest of the board and connected with vias to form both a
low thermal resistance system and to act as a ground
plane for reduced EMI.
Higher Duty Cycle
Maximum duty cycle for the LT1571-1/LT1571-2 is typi-
cally 90% but this may be too low for some applications.
For example, if an 18V 3% adapter is used to charge ten
NiMH cells, the charger must put out approximately 15V.
A total of 1.6V is lost in the input diode, switch resistance,
inductor resistance and parasitics so the required duty
12
X
P
P
P
) is from 3V to 6V.
DRIVER
DRIVER
DRIVER
V
X
BAT
VX
0 045
I
1 2
X
(see Figure 8). The optimum boost voltage
BAT
.
.
3 3
required is:
= 3.3V,
.
A
V
U
W
V
8 4
BAT
.
55
V
55 15
14
U
V
V
3 3
IN
mA
X
.
V
V
1
W
1
V
30
X
3 3
30
.
V
U
0 045
.
W
cycle is 15/16.4 = 91.4%. The duty cycle can be extended
to 93% by restricting boost voltage to 5V instead of using
V
Figure 8) also reduces power dissipation in the LT1571.
BAT
as is normally done. This lower boost voltage V
3V TO 6V
60
55
50
45
40
35
30
25
90
80
70
60
50
40
30
20
Figure 9. LT1571 Thermal Resistance
Figure 10. LT1571 Lead Temperature
V
0
0
X
I
V
V
V
T
NOTE: PEAK DIE TEMPERATURE WILL BE
ABOUT 10 C HIGHER THAN LEAD TEMPER-
ATURE AT 1.3A CHARGING CURRENT
GN16, MEASURED FROM AIR AMBIENT
TO DIE USING COPPER LANDS AS
SHOWN ON DATA SHEET
CHRG
IN
BAT
BOOST
A
= 25 C
5
= 16V
5
= 8.4V
Figure 8. Lower V
= 1.3A
I
= V
VX
L1
10
10
BAT
BOARD AREA (IN
BOARD AREA (IN
15
+
15
2-LAYER BOARD
4-LAYER BOARD
2-LAYER BOARD
4-LAYER BOARD
D2
10 F
C1
20
20
2
2
BOOST
25
25
)
)
SW
BOOST
SENSE
30
30
LT1571
1571 F09
1571 F10
35
35
1571 F08
X
(see