LTC1325CN Просмотр технического описания (PDF) - Linear Technology

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LTC1325CN

Microprocessor-controlled battery management system

Linear Technology 

LTC1325

FUNCTIONAL DESCRIPTIO

CHARGE

DR0 TO 3

DR2

DUTY RATIO

GENERATOR

111kHz

OSCILLATOR

QS

R

ONE SHOT

A2

DISCHARGE

GG

R1

R2

RF

S1 500k 125k

1k

C1

16pF

S2

S4

S3

VDD

4.5V TO 16V

PGATE

P1

IRF9Z30

RTRK

DIS

L1

D1

1N5818

BATTERY

SENSE

CF

FILTER

RDIS

N1

IRFZ34

RSENSE

TO

ADC MUX

GG VR1 VR0 DAC VOLTAGE

00 0

18mV

00 1

34mV

01 0

55mV

01 1

160mV

1X X

0mV

REG

A1

3.072V

VDAC

DAC

2

VR0, VR1

GG

CHIP

(GAS GAUGE) BOUNDARY

Figure 4. Charging Loop Block Diagram

LTC1325 • F04

rise as does the voltage across the sense resistor. When

the voltage across the sense resistor is greater than the

output of the integrator, comparator A2 changes state.

This resets the flip-flop and P1 is turned off. Catch diode

D1 clamps the drain of P1 one diode drop below ground

when the inductor flies back and the current through the

inductor starts to drop. The voltage across the sense

resistor also drops and may reach zero and stay there until

the next clock cycle begins.

The average charging current is set by the output of the

DAC (VDAC) and the duty ratio generator. VDAC can be

programmed to one of four values with the following

ratios: 1, 1/3, 1/5 or 1/10. The duty ratio can be set to

1/16, 1/8, 1/4, 1/2 or 1. When the duty ratio is 1, the duty

ratio generator output is always low and the charge loop

operates continuously (see Figure 4). At other duty ratio

settings, the duty generator output is a square wave with

a period of 42 seconds. The time for which the generator

output is low varies with the duty ratio setting. For ex-

ample, if a duty ratio of 1/2 is programmed, the generator

output is low only for 42/2 = 21 seconds. Since the loop

operates for only 21 out of every 42 seconds, the average

charging current is halved. In general, the average charg-

ing current is:

ICHRG = VDAC(Duty Ratio)/RSENSE

Gated PFET Controller

When using an external current regulator or current lim-

ited wall pack, simply remove the inductor L1 and catch

diode D1. Set the DAC control bits VR1 = 1 and VR0 = 1,

and select the desired duty ratio. By insuring that the

voltage at the Sense pin is never greater than 140mV, the

output of the integrator A1 will saturate high and the

comparator A2 will never trip and turn the loop off. This

can be achieved by removing the sense resistor and

grounding the Sense pin or if the gas gauge is to be used,

selecting RSENSE so that RSENSE/ICHRG < 140mV.

14

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