LTC4000_15 Datasheet, PDF(31/40 Page) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

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LTC4000_15 Datasheet, PDF (31/40 Pages) Linear Technology – High Voltage High Current Controller for Battery Charging and Power Management

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LTC4000

Applications Information

The Output Voltage Regulation Loop

The feedback signal for the output voltage regulation loop

is the voltage on the OFB pin, which is connected to the

center node of the resistor divider between the output

voltage (connected to CSP) and the FBG pin. This voltage

is compared to an internal reference (1.193V typical) by

the transconductance error amplifier A7. This amplifier

then drives the output transconductance amplifier (A10)

to appropriately adjust the voltage on the ITH pin driving

the external DC/DC converter to regulate the output volt-

age observed by the OFB pin. This loop is shown in detail

in Figure 21.

LTC4000

A10

gm10 = 0.1m

ITH

RO10

CSP

gm7 = 0.5m

OFB

1.193V

RO7

FBG

INTERNALLY

PULLED HIGH

INPUT

Gmop(s)

LOAD

ROFB1

CL RL

ROFB2

4000 F21

Figure 21. Simplified Linear Model of the Output Voltage

Regulation Loop

The simplified loop transmission is as follows:

LOV

(s)

gm7

ï£®

ï£¯

ï£«

ï£ï£¬

1ï£¶

gm10 ï£¸ï£·

CCs

ï£¹

1ï£º

ï£º

â¢

ï£°

ï£»

ï£®

ï£¯

ï£°

ROFB2

ROFB

ï£¹

ï£º

ï£»

â¢

ï£®

ï£°ï£¯RL

â¢ CLs

ï£¹

1ï£»ï£º

â¢ Gmop(s)

The Battery Float Voltage Regulation Loop

The battery float voltage regulation loop is very similar to

the output float voltage regulation loop. Instead of observ-

ing the voltage at the OFB pin, the battery float voltage

regulation loop observes the voltage at the BFB pin.

One significant difference is that while the value of RL

in the output voltage loop can vary significantly, the

output resistance of the battery float voltage loop is a

small constant value approximately equal to the sum

of the on-resistance of the external PFET (RDS(ON)) and

the series internal resistance of the battery (RBAT). This

approximation is valid for any efficient system such that

most of the output power from the battery is delivered to

the system load and not dissipated on the battery inter-

nal resistance or the charging PFET on-resistance. For a

typical system, minimum RL is at least five times larger

than RDS(ON) + RBAT and RBFB is at least 106 times larger

than RBAT. Figure 22 shows the detail of the battery float

voltage regulation loop.

LTC4000

A10

gm10 = 0.1m

ITH

RO10

BAT

gm6 = 0.5m

BFB

1.136V

RO6

FBG

INTERNALLY

PULLED HIGH

INPUT

Gmop(s)

LOAD

RBFB1

RBFB2

RCS

RDS(ON)

CL RL

RBAT

4000 F22

Figure 22. Simplified Linear Model of the Battery Float

Voltage Regulation Loop

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

the output current of the external DC/DC converter, and

ROFB = ROFB1 + ROFB2.

For more information www.linear.com/LTC4000

4000fb

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