LT1505 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1505' PDF : 16 Pages View PDF
LT1505
APPLICATIONS INFORMATION
With divider current set at 15µA, R4 = 2.465/15µA = 162k
and,
R3 = (R4)(VBAT − 2.465) = 162k(8.4 − 2.465)
2.465
2.465
= 390k
Li-Ion batteries typically require float voltage accuracy of
1% to 2%. Accuracy of the LT1505 VFB voltage is ±0.5%
at 25°C and ±1% over the full temperature range. This
leads to the possibility that very accurate (0.1%) resistors
might be needed for R3 and R4. Actually, the temperature
of the LT1505 will rarely exceed 50°C in float mode
because charging currents have tapered off to a low level,
so 0.25% resistors will normally provide the required level
of overall accuracy.
LT1505
VFB
R3 +
390k
0.25%
R4
162k
0.25%
VBAT
8.4V
1505 F04
Figure 4. External Resistor Divider
Lithium-Ion Charging Completion
Some battery manufacturers recommend termination of
constant-voltage float mode after charge current has
dropped below a specified level (typically around 20% of
the full current) and a further time-out period of 30
minutes to 90 minutes has elapsed. Check with manufac-
turers for details. The LT1505 provides a signal at the
FLAG pin when charging is in voltage mode and current is
reduced to 20% of full current, assuming full charge
current is programmed to have 100mV across the current
sense resistor (VRS1). The comparator E6 in the Block
Diagram compares the charge current sample IPROG to the
output current IVA voltage amplifier VA. When the charge
current drops to 20% of full current, IPROG will be equal to
0.25 IVA and the open-collector output VFLAG will go low
and can be used to start an external timer. When this
feature is used, a capacitor of at least 0.1µF is required at
the CAP pin to filter out the switching noise and a pull-up
resistor is also needed at the FLAG pin. If this feature is not
used, C6 is not needed.
Very Low Dropout Operation
The LT1505 can charge the battery even when VCC goes
as low as 0.5V above the combined voltages of the
battery and the drops on the sense resistor as well as
parasitic wiring. This low VCC sometimes requires a duty
factor greater then 99% and TGATE stays on for many
switching cycles. While TGATE stays on, the voltage
VBOOST across the capacitor C2 drops down because
TGATE control circuits require 2mA DC current. C2 needs
to be recharged before VBOOST drops too low to keep the
topside switch on. A unique design allows the LT1505 to
operate under these conditions; the comparator A2 moni-
tors VBOOST and when it drops from 8.9V to 6.9V, TGATE
will be turned off for about 0.2µs to recharge C2. Note that
the LT1505 gets started the same way when power turns
on and there is no initial VBOOST.
It is important to use 0.56µF or greater value for C2 to hold
VBOOST up for a sufficient amount of time.
When minimum operating VCC is less than 2.5V above the
battery voltage, D3 and C4 (see Figure 1) are also needed
to bootstrap VBOOSTC higher than VCC to bias the current
amplifier CA1. They are not needed if VCC is at least 2.5V
higher than VBAT. The PFET M3 is optional and can be
replaced with a diode if VIN is at least 3V higher than VBAT.
The gate control pin INFET turns on M3 when VIN gets up
above the undervoltage lockout level set by R5 and R6 and
is clamped internally to 8V below VCC. In sleep mode when
VIN is removed, INFET will clamp M3 VSG to 0.2V.
Shutdown
When adapter power is removed, VCC will drift down and
be held by the body diode of the topside NFET switch. As
soon as VCC goes down to 0.2V above VBAT, the LT1505 will
go into sleep mode drawing only 10µA from the battery.
There are two ways to stop switching: pulling the SHDN
pin low or pulling the VC pin low. Pulling the SHDN pin low
will also turn off VGBIAS and CA1 input currents. Pulling the
VC pin low will only stop switching and VGBIAS stays high.
Make sure there is a pull-up resistor on the SHDN pin even
13
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