LTC4000_15 Datasheet, PDF(33/40 Page) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

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LTC4000_15 Datasheet, PDF (33/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

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LTC4000

Applications Information

The Battery Charge Current Regulation Loop when

VOFB is Regulated to VOUT(INST_ON)

When the battery voltage is below the instant-on level,

the external charging PFET is driven linearly to regulate

the voltage at the output (connected to the CSP pin). The

output voltage is regulated such that the voltage at the OFB

pin is equal to the instant-on threshold (VOUT(INST_ON)).

If this external PFET regulation is fast compared to the

unity crossover frequency of the battery charge current

regulation loop, then the voltage at the output can be con-

sidered a small signal ground. However, in the LTC4000

the external PFET regulation is purposely made slow to

allow for a broader selection of possible PFETs to be used.

Therefore, the linear model of the PFET has to be included

in the analysis of the charge current regulation loop. The

detail of this loop is shown in Figure 24.

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

RCL

R1 R2

60k 20k

IBMON

CIBMON

CSN

BAT

4000 F24

RCS

gmEXT

RBAT

The simplified loop transmission is:

LCC2(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¢

( ) RL

RfIDC + RL

ï£®

â¢ï£¯

ï£°

RL P RfIDC

ï£¹

CLs + 1ï£»ï£º â¢

ï£®

ï£¯

ï£¯ï£«

ï£¯

ï£°ï£¯

ï£ï£¬

gmEXT

RCS + RBAT ï£¶ Cg

RfIDC ï£¸ï£· gmEXT

ï£¹

ï£º

1ï£º

ï£»ï£º

â¢

Gmop(s)

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

output current of the external DC/DC converter, gmEXT is

the small signal transconductance of the output PFET,

RflDC = RCS + 1/gmEXT + RBAT and RL//RflDC represents

the effective resistance value resulting from the parallel

combination of RL and RflDC.

Figure 24. Simplified Linear Model of the Charge

Current Regulation Loop with the External Charging

PFET Linearly Regulated

For more information www.linear.com/LTC4000

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