LT1620IGN datasheet(6/12 Pages) LINER

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LT1620/LT1621

APPLICATIONS INFORMATION

COSC

LTC1435

IN–

GND

IOUT

SENSE

IN+

LT1620MS8

VBATT

16.8V

VIN

17.3V TO 28V

LT1620/21 • F02

C11, 56pF

C12, 0.1

µF

RUN/SS

ITH

SFB

SGND

VOSENSE

SENSE–

SENSE+

BOOST

VIN

INTVCC

PGND

EXTVCC

VCC

PROG

AVG

C10

100pF

C9, 100pF

C14

1nF

C13

0.033

µF

X7R

C17, 0.01

µF

1.5M

0.1

µF

D2*

D1*

C5, 0.1

µF

Si4412DY

µH

0.1

µF

4.7

µF

RSENSE

0.025

Ω

C15

0.1

µF

C16

0.1

µF

RP1

RP2

15.75k

C18

0.1

µF

RF2

110k

0.1%

RF1

1.44M

0.1%

C8, 100pF

µF

35V

µF

35V

µF

35V

RUN

* D1, D2: CENTRAL

SEMICONDUCTOR CMDSH-3

Li-ION

Figure 2. LT1620/LTC1435 Battery Charger

Charge Current Programming

Output current delivered during current mode operation is

determined through programming the voltage at the PROG

pin (VPROG). As mentioned above, optimum performance

is obtained with (VCC – VPROG) = 0.8V. The LT1620 is

biased with a precision 5V supply produced by the LTC1435,

enabling use of a simple resistor divider from VCC to

ground for a VPROG reference. Using the desired 2.5kΩ

Thevenin impedance at the PROG pin, values of RP1 = 3k

and RP2 = 15.75k are readily calculated. The PROG pin

should be decoupled to the VCC supply.

Different values of charging current can be obtained by

changing the values of the resistors in the VPROG setting

divider to raise or lower the value of the programming

voltage, or by changing the sense resistor to an appropri-

ate value as described above.

Output Float Voltage

The 3.2A charger circuit is designed for a 4-cell Li-Ion

battery, or a battery float voltage of 16.8V. This voltage is

programmed through a resistor divider feedback to the

LTC1435 VOSENSE pin, referencing its 1.19V bandgap

voltage. Resistor values are determined through the rela-

tion: RF1 = (VBATT – 1.19)/(1.19/RF2). Setting RF2 = 110k

yields RF1 = 1.44M.

Other Decoupling Concerns

The application schematic shown in Figure 2 employs

several additional decoupling capacitors. Due to the inher-

ently noisy environment created in switching applications,

decoupling of sensitive nodes is prudent. As noted in the

schematic, decoupling capacitors are included on the

current programming pin (PROG) to the VCC rail and

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