LTC4000
27
4000fb
For more information
applicaTions inForMaTion
• The charge termination time is set at 2.9 hours accord-
ing to the following formula:
C
TMR
(nF)
=
t
TERMINATE
(h) • 34.6
=
2.9 • 34.6
=
100nF
• The C/X current termination level is programmed at 1A
according to the following formula:
R
CX
=
1A • 5m
Ω
(
)
+
0.5mV
0.25µA
≈
22.1k
Ω
Note that in this particular solution, the timer termina-
tion is selected since a capacitor connects to the TMR
pin. Therefore, this C/X current termination level only
applies to the
CHRG
indicator pin.
• The output voltage regulation level is set at 12V accord-
ing to the following formula:
R
OFB1
=
12
1.193
−
1
• 127k
Ω ≈
1.15M
Ω
• The instant-on voltage level is consequently set at 9.79V
according to the following formula:
V
INST _ ON
=
1150k
Ω +
127k
Ω
127k
Ω
• 0.974V
=
9.79V
The worst-case power dissipation during instant-on
operation can be calculated as follows:
• During trickle charging:
P
TRKL
=
0.86 • V
FLOAT
– V
BAT
•I
CLIM _ TRKL
=
0.86 • 10.8
[
]
• 1A
=
9.3W
• And beyond trickle charging:
P
INST _ ON
=
0.86 • V
FLOAT
– V
BAT
•I
CLIM
=
0.86 • 10.8 – 7.33
[
]
• 10A
=
19.3W
Therefore, depending on the layout and heat sink avail-
able to the charging PMOS, the suggested PMOS over
temperature detection circuit included in Figure 7 may
need to be included. For the complete application circuit,
please refer to Figure 25.
• The range of valid temperature for charging is set at
–1.5°C to 41.5°C by picking a 10k Vishay Curve 2 NTC
thermistor that is thermally coupled to the battery, and
connecting this in series with a regular 10k resistor to
the BIAS pin.
• For compensation, the procedure described in the
empirical loop compensation section is followed. As
recommended, first a 1µF C
C
and 10k R
C
is used, which
sets all the loops to be stable. For an example of typical
transient responses, the charge current regulation loop
when V
OFB
is regulated to V
OUT(INST_ON)
is used here.
Figure 15 shows the recommended setup to inject a
DC-coupled charge current variation into this particular
loop. The input to the CL pin is a square wave at 70Hz
with the low level set at 120mV and the high level set
at 130mV, corresponding to a 1.2A and 1.3A charge
current (100mA charge current step). Therefore, in this
particular example the trickle charge current regulation
stability is examined. Note that the nominal trickle
charge current in this example is programmed at 1.25A
(R
CL
= 24.9kΩ).
Figure 15. Charge Current Regulation Loop Compensation Setup
LTC4000
10k
1k
4000 F15
0.015µF
SQUARE WAVE
GENERATOR
f = 60Hz
IBMON
CL
1500pF
B
A