RT9451 datasheet(29/37 Pages) RICHTEK

29 / 37 page

RT9451

DS9451-00

September 2013

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

Fast-Charge Mode and Settings

As the BATS pin rises above VPREC, the charger enters

fast-charge mode and the power MOSFETs starts

switching. Unlike the linear charger, the switching charger

is a current amplifier. The current drawn by the RT9451 is

different from the current into the battery. The user can

set the input current limit (IIN_LIMIT) and output charge

current (ICHRG) respectively. The real charging current is

determined by both IIN_LIMIT [1:0] and VICHRG [3:0].

Cycle-by-Cycle Current Limit

The charger of the RT9451 has an embedded cycle-by-

cycle current limit for inductor. Once the inductor current

touches the threshold (5.5A min.), the charger stops

charging immediately to prevent over current from damaging

the device. Notice that, the mechanism can not be disabled

by any way.

Low Current Charging (LOW_CHG)

The RT9451 has two charge current settings for the charge

current in fast-charge mode. One is LOW_CHG bit, the

other is VICHRG[3:0] bits. If LOW_CHG is set to

“1”,

the charge current is limited to 150mA (RSENSE = 20m

Ω).

If LOW_CHG is set to

“0”, the charge current is

determined by VICHRG[3:0] bits.

Charge Current (ICHRG)

The charge current into the battery is determined by the

sense resistor (RSENSE) and VICHRG [3:0] bits in the

and ISENR pins is regulated to the voltage controlled by

VICHRG [3:0]. The charge current equals to the voltage

between the ISENL and ISENR pins (VICHRG) divided by

RSENSE :

ICHRG

CHRG

SENSE

For example, for a 20m

Ω sense resistor, the charge current

can be set from 1000mA (VICHRG [3:0] =

“0000”) to

4000mA (VICHRG [3:0] =

“1111”).

When input current limit and charge current are both set,

the charge current in fast charge phase is calculated as

below :

IN_LIMIT

ICHRG

CHRG

SENSE

= MIN

⎡⎤

⎛⎞

⎢⎥

⎜⎟

⎝⎠

⎣⎦

where D is the duty cycle and

η is the efficiency.

Constant Voltage Mode and Settings

The RT9451 enters constant voltage mode when the BATS

voltage is close to the output-charge voltage (VOREG). Once

entering this mode, the charge current begins decreasing.

For default settings (charge current termination is disabled,

TERM_EN = 0), the RT9451 does not turn off and always

regulates the battery voltage at VOREG. However, once

the charge current termination is enabled, the charger

terminates if the charge current is below termination

current (VITERM [2:0] / RSENSE) in constant-voltage mode.

The charge current termination function is controlled by

TERM_EN bit in the register of address 0x01 via the I

interface. After termination, a new charge cycle restarts

when one of the following conditions is detected :

The BATS pin voltage falls below the VOREG

−VRECH

threshold and CH_EN [1:0] = 11.

VIN Power On Reset (POR).

CH_EN [1:0] is set from

“0X” to “1X”.

Output Charge Voltage (VOREG)

The output-charge voltage is set by the VOREG [5:0] in

the register 0x02h. Its range is from 3.5V to 4.45V. The

default is 4V (011001).

Termination Current (IEOC)

If the charger current termination is enabled (TERM_EN

“1”), the end-of-charge current is determined by both

the termination current sense voltage (VITERM [2:0]) and

sense resistor (RSENSE). VITERM range is from 1mV to

8mV with the step of 1mV. The end-of-charge current is

calculated as below :

ITERM

EOC

SENSE

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