LTC4101EG Просмотр технического описания (PDF) - Linear Technology

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LTC4101EG

Smart Battery Charger Controller

Linear Technology 

LTC4101

APPLICATIONS INFORMATION

The Schottky diode D1, shown in the Typical Application

on the back page, conducts during the dead-time between

the conduction of the two power MOSFETs. This prevents

the body diode of the bottom MOSFET from turning on and

storing charge during the dead-time, which could cost as

much as 1% in efficiency. A 1A Schottky is generally a good

size for 4A regulators due to the relatively small average

current. Larger diodes can result in additional transition

losses due to their larger junction capacitance.

The diode may be omitted if the efficiency loss can be

tolerated.

Calculating IC Power Dissipation

The power dissipation of the LTC4101 is dependent upon

the gate charge of the top and bottom MOSFETs (Q2 &

Q3 respectively) The gate charge (QG) is determined from

the manufacturer’s data sheet and is dependent upon both

the gate voltage swing and the drain voltage swing of the

MOSFET. Use 6V for the gate voltage swing and VDCIN for

the drain voltage swing.

PD = VDCIN • (fOSC (QGQ2 + QGQ3) + IDCIN) + VDD • IDD

Example: VDCIN = 12V, fOSC = 345kHz, QGQ2 = 25nC,

QGQ3 = 15nC, IDCIN = 5mA, VDD = 5.5V,

IDD = 1mA.

PD = 231mW

Calculating VDD Current

The LTC4101 VDD current, or IDD, consist of three parts:

a. IRUN = Current due to active clocking and bias inside

the IC.

b. ITHRM = Current due to thermistor circuit activity.

c. IACCEL = Current due to SMBus acceleration activity.

IDD = IRUN + ITHRM + IACCEL

a) IRUN current is basically independent of SCL clock rate.

Once the LTC4101 determines that there is activity on

the SMBus, it turns on its internal HF oscillator. This

HF oscillator remains on until a stop event occurs or

SDA and SCL are at logic level 1 for the SMBus timeout

period. Then it shuts off the HF oscillator. Thus, the

length of the transmission and the rate of transmission

bursts are more important in determining how much

24

current the LTC4101 burns, rather than the SCL rate.

In the equation below, IQ is the static current the IC

consumes as a function of the VDD voltage when not

active. Since it is hard to quantify the actual messages

going down the SMBus, one must estimate the SMBus

activity level in term of bus utilization per second.

IRUN = Message Duty Cycle • 950μA

+ (1 – Message Duty Cycle) • IQ

where IQ(TYP) = VDD/47.2k

b) ITHRM current is due to SafetySignal (thermistor pin)

sampling that will vary with the presence of DC power

being on or off. DCDIV is detected every 32ms. RTHX

is the value of the safety signal resistance, which will

vary with temperature or battery configuration.

b1) ITHRM(ON) when DC is on:

ITHRM(ON)_OVERRANGE = 1/16 • VDD/(54.9k + RTHX)

where RTHX > 100k

ITHRM(ON)_COLD = 1/8 • VDD/(54.9k + RTHX)

where RTHX > 30k

ITHRM(ON)_NORMAL = 1/8 • VDD/(54.9k + RTHX)

+ 1/16 • VDD/(1.13k + RTHX)

ITHRM(ON)_HOT* = 1/8 • VDD/(54.9k + RTHX) + 1/8

• VDD/(1.13k + RTHX) RTHX < 3k

* includes underrange

b2)ITHRM(OFF) when DC is off, the thermistor monitoring

rate is reduced to every 250ms or less.

ITHRM(OFF)_OVERRANGE = 1/50 • VDD/(54.9k + RTHX)

where RTHX > 100k

ITHRM(OFF)_COLD = 1/50 • VDD/(54.9k + RTHX)

+ 1/1000 • VDD/(54.9k + RTHX) RTHX > 30k

ITHRM(OFF)_NORMAL = 1/50 • VDD/(54.9k + RTHX)

+ 1/500 • VDD/(54.9k + RTHX) + 1/1000

• VDD/(1.13k + RTHX)

ITHRM(OFF)_HOT* = 1/50 • VDD/(54.9k + RTHX)

+ 1/500 • VDD/(54.9k + RTHX) + 1/500

• VDD/(1.13k + RTHX)

where RTHX < 3k

* includes underrange

4101fa

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