LT1769IGN View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LT1769IGN
Linear Technology
'LT1769IGN' PDF : 16 Pages View PDF
LT1769
TEST CIRCUITS
VC
0.047µF
1k
+
≈ 0.65V
Test Circuit 1
LT1769
CA2
CA1
60k
1k
VREF
PROG
+
LT1006
–
1µF
300Ω
RPROG
20k
SPIN
RS3
SENSE 200Ω
RS2
BAT 200Ω
RS1
100Ω
VBAT
1769 TC01
Test Circuit 2
LT1769
VA
IPROG
PROG
10k
VREF
10k
–
0.47µF RPROG
LT1013
+
+
2.465V
OVP
1769 TC02
U
OPERATION
The LT1769 is a current mode PWM step-down (buck)
switcher. The battery DC charge current is programmed
by a resistor RPROG (or a DAC output current) at the PROG
pin (see Block Diagram). Amplifier CA1 converts the
charge current through RS1 to a much lower current IPROG
fed into the PROG pin. Amplifier CA2 compares the output
of CA1 with the programmed current and drives the PWM
control loop to force them to be equal. High DC accuracy
is achieved with averaging capacitor CPROG. Note that
IPROG has both AC and DC components. IPROG goes
through R1 and generates a ramp signal that is fed to the
PWM control comparator C1 through buffer B1 and level
8
shift resistors R2 and R3, forming the current mode inner
loop. The BOOST pin drives the switch NPN QSW into
saturation and reduces power loss. For batteries like
lithium-ion that require both constant-current and con-
stant-voltage charging, the 0.5%, 2.465V reference and
the amplifier VA reduce the charge current when battery
voltage reaches the preset level. For NiMH and NiCd, VA
can be used for overvoltage protection. When the input
voltage is removed, the VCC pin drops to 0.7V below the
battery voltage, forcing the charger into a low battery drain
(3µA typical) sleep mode. To shut down the charger,
simply pull the VC pin low with a transistor.
1769fa
Share Link: 