SC804 datasheet(8/21 Pages) SEMTECH

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SC804

PRELIMINARY

POWER MANAGEMENT

Fast-Charge Mode (CC)

The fast-charge CC (Constant Current) mode is active when

the battery voltage is above VT

PreQ and less than VCV. The

fast-charge current can be set to a maximum of 1.5A and

is selected by the program resistor on the IPRGM pin. The

voltage on this pin will represent the current through the

battery, enabling a microprocessor via an analog-to-digital

converter (ADC) to monitor battery current by sensing the

voltage on the IPRGM pin. The equation to set the fast-

charge current is given by:

The superior fast-charge current accuracy of the SC804

is obtained by use of a patented* polarity-switched (i.e.,

chopped) current sense ampliﬁer to nullify current mea-

surement offset errors.

Compliance with the absolute maximum output current

VOUTMAX, allowing for current regulation tolerance, requires

that R

IPRGM be no smaller than 1.05kΩ nominal.

IPRGM

can be as large as 11.5k

Ω, for a nominal FCI as small

as 130mA, but must exceed PCI by at least 80mA. Note

that for a given program resistor the current through

the battery in CV mode can be determined by replacing

IPRGM_Typ with the actual voltage on the IPRGM pin in the

above equation. The CC current can also be modiﬁed by

applying an analog voltage to the AFC pin as described

below.

Analog Fast Charge (AFC Pin)

Many applications require more than one current setting

for fast-charge. This behavior is obtained in the SC804

using the AFC function. When the AFC pin is connected

to VCC the device behaves as described in the previous

section. When the AFC pin is driven by an analog voltage

between 0V and (V

VCC-1.0)V, the SC804 automatically uses

this pin voltage to set the maximum fast-charge current

according to the following equation:

General Operation

The SC804 can be conﬁgured independently with respect

to fast-charge and termination current, output voltage,

and timing, depending on the application. A typical

charging cycle is described below. Details on alternative

applications and output programmability are covered in

the individual sections.

The charging cycle begins when the power adapter is

connected to the device. The SC804 performs glitch

ﬁltering on the VCC input and initiates a charge cycle when

VCC is greater than the under-voltage lockout (UVLO) rising

threshold voltage. If the battery voltage is less than the

pre-charge threshold level, the SC804 will output the pre-

charge current. Once the pre-charge threshold voltage

is exceeded, the SC804 enters fast-charge constant

current (CC) mode. When the battery voltage reaches its

ﬁnal value, the charger enters the constant voltage (CV)

mode. In this mode the output current decreases as the

battery continues to charge until the termination current

level is reached. The CHRGB output turns off when IOUT

drops below the termination current. If the charge timer

is active, the SC804 continues to hold the battery in CV

charge mode until the timer expires. When the timer

expires the charger enters the monitor mode where the

output remains off until the voltage at VOUT drops by

ReQ. At this point a new charge cycle is initiated.

Pre-Charge Mode

Pre-charge mode is automatically enabled whenever

the battery voltage is below the pre-charge threshold

voltage, VT

PreQ. It is used to limit the power dissipation and

precondition the battery for fast charging. The pre-charge

current value is determined by the resistor on the ITERM

pin. The pre-charge current is programmable from 50mA

to 350mA. The equation to select the pre-charge current

is given by:

Where V

ITERM_Typ designates the typical value of VITERM. When

the timer is enabled there is also a maximum allowed pre-

charge duration. If the pre-charge time exceeds 25% of

the total charge cycle the charger will turn off due to a

pre-charge fault. This fault is cleared when VCC is toggled

or the output voltage rises above VT

PreQ.

PCI =

ITERM_Typ

× 100

ITERM

IPRGM_Typ

× 1000

FCI =

IPRGM

FCI =

AFC

× 1000

IPRGM

Applications Information

*US Patent 6,836,095.

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