Datasheet

14
LT1511
APPLICATIONS INFORMATION
WUU
U
low thermal resistance system and to act as a ground
plane for reduced EMI.
Glue-on, chip-mounted heat sinks are effective only in
moderate power applications where the PC board copper
cannot be used, or where the board size is small. They
offer very little improvement in a properly laid out multi-
layer board of reasonable size.
Higher Duty Cycle for the LT1511 Battery Charger
Maximum duty cycle for the LT1511 is typically 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 15V maximum. A total of 1.6V is lost
in the input diode, switch resistance, inductor resistance
and parasitics, so the required duty cycle is 15/16.4 =
91.4%. As it turn out, duty cycle can be extended to 93%
by restricting boost voltage to 5V instead of using V
BAT
as
is normally done. This lower boost voltage also reduces
power dissipation in the LT1511, so it is a win-win deci-
sion. Connect an external source of 3V to 6V at V
X
node in
Figure 10 with a 10µF C
X
bypass capacitor.
Even Lower Dropout
For even lower dropout and/or reducing heat on the board,
the input diode D3 should be replaced with a FET (see
Figure 11). It is pretty straightforward to connect a
P-channel FET across the input diode and connect its gate
to the battery so that the FET commutates off when the
input goes low. The problem is that the gate must be
pumped low so that the FET is fully turned on even when
the input is only a volt or two above the battery voltage.
Also there is a turn-off speed issue. The FET should turn
Figure 9. LT1511 Lead Temperature
BOARD AREA (IN
2
)
0
45
40
35
30
25
20
15
10
15 25
LT1511 • F08
510
20 30 35
THERMAL RESISTANCE (°C/W)
MEASURED FROM AIR AMBIENT
TO DIE USING COPPER LANDS
AS SHOWN ON DATA SHEET
2-LAYER BOARD
4-LAYER BOARD
Figure 8. LT1511 Thermal Resistance
BOARD AREA (IN
2
)
0
110
100
90
80
70
60
50
40
15 25
LT1511 • F09
510
20 30 35
LEAD TEMPERATURE (°C)
V
IN
= 16V
V
BAT
= 8.4V
I
CHRG
= 3A
T
A
= 25°C
NOTE: PEAK DIE TEMPERATURE WILL BE
ABOUT 10°C HIGHER THAN LEAD TEMPER-
ATURE AT 3A CHARGING CURRENT
2-LAYER BOARD
4-LAYER BOARD
4-LAYER BOARD
WITH V
BOOST
= 3.3V
SW
BOOST
SPIN
SENSE BAT
V
BAT
C3
0.47µF
D2
LT1511
SW
BOOST
SPIN
SENSE BAT
V
X
3V TO 6V
C
X
10µF
V
BAT
1511 F10
C3
0.47µF
D2
LT1511
STANDARD CONNECTION HIGH DUTY CYCLE CONNECTION
+ +
V
IN
SW
BOOST
SPIN
SENSE BAT
V
CC
V
X
3V TO 6V
C
X
10µF
V
BAT
1511 F11
C2
0.47µF
D2
D1
R
X
50k
Q2
Q1
LT1511
HIGH DUTY CYCLE CONNECTION
Q1 = Si4435DY
Q2 = TP0610L
+
+
Figure 11. Replacing the Input Diode
Figure 10. High Duty Cycle