SC804A datasheet(10/21 Pages) SEMTECH

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SC804A

PRELIMINARY

POWER MANAGEMENT

DRAFT

or R3 = 2.333×R

HOT = 13.624kΩ exactly. The closest 1%

standard nominal value is R3 = 13.7k

Ω.

Step 2: Verify acceptable thermistor self heating. In

general, lower values of RT provide more noise immunity

for the NTC voltage, but at the expense of bias current

from the input adapter and power dissipation in the NTC

network. The dissipation constant is the power rating of

the thermistor resulting in a 1oC self heating error. The

greatest self-heating occurs at low thermistor resistance

(at high temperature).

Since temperature sensing

accuracy matters only at the charging temperature range

thresholds, self heating is assessed only at the worst case

high temperature threshold of +40oC.

For V

VCC = 5V,

the 40oC NTC network current I

NTC_HOT =

VCC /(R3 + RHOT) = 0.246mA.

Power dissipation in the

thermistor at this temperature, P

HOT = RHOT × (INTC_HOT)

0.38mW, for self heating of approximately 0.13oC. The

actual high temperature threshold will thus be lower by

0.13oC. This self-heating error is usually acceptable. If

it is not, then a thermistor with a greater R

HOT must be

chosen.

Step 3: Determine the desired high (cold) threshold.

Compute the NTC network resistor divider voltage, as a

function of V

VCC, at the cold temperature threshold.

Step 4: Conﬁgure CTO. If NTC

COLD is sufﬁciently close

to the default cold threshold (VT

NTCC×V

VCC), then simply

connect CTO to ground, disabling the CTO function, to

complete the design. But in this example it is not, so the

voltage on CTO must be set to 0.6591×V

VCC. The simple

resistive voltage divider network of Figure 2 can be used

to obtain the desired CTO voltage.

Applications Information (Cont.)

the SC804A and resets the charge timer (with the FLTB

pin inactive). The NTC pin can be pulled down to ground

by an external n-channel FET transistor or processor GPIO

to disable or reset the SC804A.

Note that the response of the SC804A to NTC pin voltage

above the high threshold and below the low threshold

is the same. Thus it is possible to conﬁgure the NTC

network with the battery pack thermistor between NTC

and VCC, and a ﬁxed resistor between NTC and ground.

This conﬁguration may be useful if it is desired to reset the

charge timer (and the CHRGB output) when the battery

pack is removed (so the ﬁxed resistor pulls the NTC pin to

ground) while VCC is present.

Cold Temperature Offset (CTO)

The voltage applied to the CTO pin sets the NTC high

voltage (normally the cold temperature threshold) for the

NTC input. The default NTC high threshold (VT

NTCC×V

VCC)

can be selected by connecting the CTO pin to ground. If

it is desired to change this threshold, the voltage on the

CTO pin can be set between 0.5×V

VCC and 0.9×VVCC.

This feature is especially useful if a single PCB design

is needed to satisfy similar applications with different

requirements. The temperature range for normal charging

can be adjusted by adjusting resistor values on a divider

network without changing the NTC thermistor, which

is often enclosed in the battery pack. An example of a

typical application is shown in Figure 2.

NTC/CTO Design Example

The following example assumes the NTC network

conﬁguration of Figure 2, with a ﬁxed resistor R3 connected

between NTC and VCC, and a battery NTC thermistor

RT connected between NTC and ground. The battery

temperature range over which charging is permitted is

0oC through 40oC. The datasheet for the selected NTC

thermistor indicates that RT = 5.839k

Ω at 40oC, at RT =

26.49k

Ω at 0oC, with a dissipation constant DC = 3mW.

Designate R

HOT = 5.839kΩ and RCOLD = 26.49kΩ.

Step 1: Select R3. For the normal (NTC thermistor to

ground) conﬁguration, solve the NTC network voltage

divider for R3 to place the NTC voltage at 0.3×VCC when

RT = R

HOT.

0.3 × VCC =

VCC × R

HOT

R3 + R

HOT

NTC

COLD =

VCC × R

COLD = 0.6591 × VCC

R3 + R

COLD

CTO = NTCCOLD

= 0.6591 × VCC =

VCC × R

CT2

CT1 + RCT2

CT1 = 1 0.6591 = 0.5172

CT2

0.6591

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