LT1769
U
W U U
APPLICATIONS INFORMATION
VUV = Rising lockout threshold on the UV pin
adapter load current remains below the limit. Amplifier
CL1 in Figure 2 senses the voltage across RS4, connected
betweentheCLPandCLNpins. Whenthisvoltageexceeds
100mV,theamplifierwilloverridetheprogrammedcharge
current to limit adapter current to 100mV/RS4. A lowpass
filter formed by 500Ω and 1µF is required to eliminate
switching noise. If the input current limit is not used, both
CLP and CLN pins should be connected to VCC.
VIN =Chargerinputvoltagethatwillsustainfullloadpower
Example: With R6 = 5k, VUV = 6.7V and setting VIN at 12V;
R5 = 5k (12V – 6.7V)/6.7V = 4k
The resistor divider should be connected directly to the
adapter output as shown, not to the VCC pin, to prevent
battery drain with no adapter voltage. If the UV pin is not
used, connect it to the adapter output (not VCC) and
connect a resistor no greater than 5k to ground. Floating
this pin will cause reverse battery current to increase from
3µA to 200µA.
Charge Current Programming
The basic formula for charge current is (see Block
Diagram):
IfconnectingtheunusedUVpintotheadapteroutputisnot
possible, it can be grounded. Although it would seem that
grounding the pin creates a permanent lockout state, the
UV circuitry is arranged for phase reversal with low volt-
agesontheUVpintoallowthegroundingtechniquetowork.
R
R
2.465V R
S2
S1
S2
S1
I
= I
=
BAT
PROG
(
) (
) (
)
R
R
PROG
whereRPROG isthetotalresistancefromPROGpintoground.
For the sense amplifier CA1 biasing purpose, RS3 should
have the same value as RS2 and SPIN should be connected
directly to the sense resistor (RS1) as shown in the Block
Diagram.
100mV
CLP
+
+
1µF
CL1
500Ω
CLN
For example, 2A charge current is needed. For low power
dissipation on RS1 and enough signal to drive the amplifier
CA1, let RS1 = 100mV/2A = 0.05Ω. This limits RS1 power
to 0.2W. Let RPROG = 5k, then:
–
AC ADAPTER
OUTPUT
R
*
V
S4
CC
V
IN
+
R5
LT1769
UV
*R
(I )(R
)(R )
S1
BAT
PROG
R
S2
= R
=
S3
R6
100mV
ADAPTER CURRENT LIMIT
=
2.465V
S4
1769 F02
(2A)(5k)(0.05)
2.465V
=
= 200Ω
Figure 2. Adapter Input Current Limiting
Charge current can also be programmed by pulse width
modulatingIPROG withaswitchQ1toRPROG atafrequency
higher than a few kHz (Figure 3). Charge current will be
proportionaltothedutycycleoftheswitchwithfullcurrent
at 100% duty cycle.
Adapter Current Limiting
An important feature of the LT1769 is the ability to
automaticallyadjustchargecurrenttoalevelwhichavoids
overloading the wall adapter. This allows the product to
operate at the same time the batteries are being charged
without complex load management algorithms. Addition-
ally, batteries will automatically be charged at the maximum
possible rate of which the adapter is capable.
Lithium-Ion Charging
The 2A Lithium-Ion Battery Charger (Figure 1) charges at
aconstant2AuntilbatteryvoltagereachesalimitsetbyR3
and R4. The charger will then automatically go into a
constant-voltage mode with current decreasing to near
zeroovertimeasthebatteryreachesfullcharge. Thisisthe
This is accomplished by sensing total adapter output
current and adjusting the charge current downward if a
preset adapter current limit is exceeded. True analog
control is used, with closed-loop feedback ensuring that
normal regimen for lithium-ion charging, with the charger
1769fa
10
Linear Systems [ Linear Systems ]
