LT1505 datasheet(13/16 Pages) LINER

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LT1505

With divider current set at 15

µA, R4 = 2.465/15µA = 162k

and,

R4 V

2.465

162k 8.4

2.465

390k

BAT

= ()

−

()

−

()

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.

APPLICATIONS INFORMATION

VBAT

1505 F04

8.4V

390k

0.25%

162k

0.25%

VFB

LT1505

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 0.1

µF 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 0.2

µs 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 0.56

µF 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 10

µA 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

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