LT1511CSW View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT1511CSW
Linear Technology
'LT1511CSW' PDF : 16 Pages View PDF
LT1511
APPLICATIONS INFORMATION
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 VBAT 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 VX node in
Figure 10 with a 10µF CX 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
45
40
35
2-LAYER BOARD
30
4-LAYER BOARD
25
20
MEASURED FROM AIR AMBIENT
15 TO DIE USING COPPER LANDS
AS SHOWN ON DATA SHEET
10
0 5 10 15 20 25
BOARD AREA (IN2)
30 35
LT1511 • F08
Figure 8. LT1511 Thermal Resistance
110
NOTE: PEAK DIE TEMPERATURE WILL BE
100 ABOUT 10°C HIGHER THAN LEAD TEMPER-
ATURE AT 3A CHARGING CURRENT
90
2-LAYER BOARD
80
70
4-LAYER BOARD
60
VIN = 16V
4-LAYER BOARD
50
VBAT = 8.4V
ICHRG = 3A
WITH VBOOST = 3.3V
TA = 25°C
40
0 5 10 15 20 25 30 35
BOARD AREA (IN2)
LT1511 • F09
Figure 9. LT1511 Lead Temperature
14
STANDARD CONNECTION
HIGH DUTY CYCLE CONNECTION
C3
0.47µF
D2
SW
BOOST
LT1511
SPIN
SENSE BAT
+
C3
0.47µF
D2
VX
3V TO 6V
CX
10µF
VBAT
SW
BOOST
LT1511
SPIN
SENSE BAT
VBAT
+
Figure 10. High Duty Cycle
HIGH DUTY CYCLE CONNECTION
VIN
Q1
+
RX
50k
Q1 = Si4435DY
Q2 = TP0610L
Q2
C2
D1
0.47µF
D2
VX
3V TO 6V
CX
10µF
SW VCC
BOOST
LT1511
SPIN
SENSE BAT
+
Figure 11. Replacing the Input Diode
1511 F10
VBAT
1511 F11
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