LT1571 Просмотр технического описания (PDF) - Linear Technology

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LT1571

Constant-Current/Constant-Voltage Battery Charger with Preset Voltage and Termination Flag

Linear Technology 

LT1571 Series

APPLICATIO S I FOR ATIO

C1 D1

0.22µF 1N5819

SW

VCC

D3

1N5819

CIN*

10µF

VIN

(WALL ADAPTER)

L1**

33µH

D2

1N914

BOOST PROG

LT1571-1

GND

VC

1µF

300Ω

R1

100k

0.1µF

1k

IBAT

SENSE BAT +

* TOKIN OR MARCON CERAMIC

SURFACE MOUNT

** COILTRONICS CTX33-2

+

COUT

22µF

TANT

2V TO

20V

R2

11k

Q1

VN2222

ON: IBAT = 0.5A

OFF: IBAT = 0.05A

1571 F07

Figure 7. Charging NiMH or NiCd Batteries with

Constant Current (Efficiency at 0.5A ≈ 90%)

lower value and maintained as a constant trickle charge.

An intermediate “top off” current may also be used for a

fixed time period to reduce total charge time.

NiMH batteries are similar in chemistry to NiCd but have

two differences related to charging. First, the inflection

characteristic in battery voltage as full charge is

approached is not nearly as pronounced. This makes it

more difficult to use – dV/dt as an indicator of full charge,

and an increase in temperature is more often used with a

temperature sensor located in the battery pack. Secondly,

constant trickle charge may not be recommended. In-

stead, a moderate level of current is used on a pulse basis

(≈ 1% to 5% duty cycle) with the time-averaged value

substituting for a constant low trickle.

Thermal Calculations

If the LT1571 is used for charge currents above 0.4A, a

thermal calculation should be done to ensure that junction

temperature will not exceed 125°C. Power dissipation in

the IC is caused by bias and driver current, switch resis-

tance, switch transition losses and the current sense

resistor. The following equations show that maximum

practical charge current for the 16-pin SSOP package

(75° C/W thermal resistance) is about 1.2A for an 8.4V

battery and 1.4A for a 4.2V battery. This assumes a 60°C

maximum ambient temperature. The 28-pin SSOP, with a

thermal resistance of 40°C/W, can provide a full 1.5A

charge current in many situations.

PBIAS = (3.5mA)(VIN) + 1.5mA(VBAT)

[ ] + (VBAT)2

VIN

7.5mA + (0.012)(IBAT)

PDRIVER

=

(IBAT )(VBAT )2 1+

55(VIN)

VBAT

30



PSW

=

(IBAT )2 (RSW )(VBAT )

VIN

+

(tOL)(VIN)(IBAT)(f)

PSENSE = (0.18Ω)(IBAT)2

RSW = Switch ON resistance ≈ 0.35Ω

tOL = Effective switch overlap time ≈ 10ns

f = 200kHz (500kHz for LT1571-5)

Example: VIN = 15V, VBAT = 8.4V, IBAT = 1.2A;

PBIAS = (3.5mA)(VIN) + 1.5mA(VBAT)

[ ] + (VBAT)2

VIN

7.5mA + (0.012)(IBAT)

PDRIVER

=

(IBAT )(VBAT )2 1+

55(VIN)

VBAT

30



PSW

=

(IBAT )2 (RSW )(VBAT )

VIN

+

(tOL)(VIN)(IBAT)(f)

PSENSE = (0.18Ω)(IBAT)2

Total power in the IC is:

0.17 + 0.13 + 0.32+ 0.26 = 0.88W

Temperature rise will be (0.88W)(40°C/W) = 35°C. This

assumes that the LT1571 is properly heat sunk by con-

necting all fused ground pins to the expanded traces and

that the PC board has a backside or internal plane for heat

spreading.

The PDRIVER term can be reduced by connecting the boost

diode D2 to a lower system voltage (lower than VBAT)

11

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