LTC4000_15 Datasheet, PDF(32/40 Page) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

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LTC4000_15 Datasheet, PDF (32/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

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LTC4000

Applications Information

In Figure 22 the battery is approximated to be a signal

ground in series with the internal battery resistance RBAT.

Therefore, the simplified loop transmission is as follows:

LBV

(s)

gm6

ï£®

ï£¯

ï£«

ï£ï£¬

1ï£¶

gm10 ï£¸ï£·

CCs

1ï£¹ï£º

ï£º

â¢

ï£°

ï£»

ï£®

ï£¯

ï£°

RBFB2

RBFB

ï£¹

ï£º

ï£»

â¢

ï£®

ï£°ï£¯ RLB

RLB

â¢ CLs

ï£¹

1ï£»ï£º

â¢

Gmop(s)

where Gmop(s) is the transfer function from VITH to

the output current of the external DC/DC converter,

RBFB = RBFB1 + RBFB2, and RLB = RL//(RDS(ON) + RCS +

RBAT) represents the effective output resistance from the

LOAD node to GND.

The Battery Charge Current Regulation Loop

This final regulation loop combines certain dynamic char-

acteristics that are found in all the other three loops. The

feedback signal for this charge current regulation loop is

the sense voltage across the charge current sense resis-

tor (RCS). This voltage is amplified by a factor of 20 and

compared to the voltage on the CL pin by the transcon-

ductance error amplifier (A5). In a familiar fashion, this

amplifier drives the output transconductance amplifier

(A10) to appropriately adjust the voltage on the ITH pin

driving the external DC/DC converter to regulate the input

current across the sense resistor (RCS).

Due to the presence of the instant-on feature, description

of the charge current regulation loop has to be divided into

two separate operating regions. These regions of operation

depend on whether the voltage on the OFB pin is higher

or lower than the instant-on threshold (VOUT(INST_ON)).

The Battery Charge Current Regulation Loop when

VOFB > VOUT(INST_ON)

In this operating region, the external charging PFETâs gate

is driven low and clamped at VBGATE(ON). The detail of this

loop is shown in Figure 23.

The simplified loop transmission is:

LCC(s)

gm5

ï£®

ï£¯

ï£«

ï£ï£¬

1ï£¶

gm10 ï£¸ï£·

CCs

ï£¹

1ï£º

ï£º

â¢

ï£°

ï£»

( ) 20RCS

â¢

ï£®

ï£¯

ï£°

(R1+

â¢ CIBMONs + 1

R2)CIBMONs +

ï£¹

1ï£»ï£º

â¢

Rf + RL

â¢

ï£®

ï£¯

ï£°

(RL

Rf )CLs

ï£¹

1ï£»ï£º

â¢

Gmop(s)

where Gmop(s) is the transfer function from VITH to the

output current of the external DC/DC converter, Rf =

RCS + RDS(ON) + RBAT, and RL//Rf represents the effective

resistance value resulting from the parallel combination

of RL and Rf.

RC CC

RO10

ITH

A10

gm10 = 0.1m

LTC4000

INPUT

Gmop(s)

RO5

gm5 = 0.5m

gm8 = 0.33m CSP

CL RL

BIAS 1V

50ÂµA/

5ÂµA

RCS

R1 R2

CSN

60k 20k

RDS(ON)

IBMON

BAT

4000 F23

RBAT

RCL

CIBMON

Figure 23. Simplified Linear Model of the Charge Current

Regulation Loop with the External Charging PFET Driven On

4000fb

For more information www.linear.com/LTC4000

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