SC805 datasheet(8/15 Pages) SEMTECH

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 2005 Semtech Corp.

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SC805

POWER MANAGEMENT

Step 3: Calculate the Tcold value.

2.86

10K

COLD

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The ratio for the curve 2 NTC at 0oC is 2.816. The differ-

ence represents less than 1/2oC (i.e., actual trip would

be -0.5oC instead of 0oC.) For temperature trip values

other than 0oC and 50oC alternate curve-type NTC ele-

ments can be used or an additional resistor can be added

to the Rntc divider to alter the temperture coefficient.

CHRGB LED Flag

The CHRGB LED driver on the SC805 can be used to

drive two external LEDs: CHRGB=Low (Charging) and

CHRGB=High (Charge Complete).

The CHRGB output

will be high impedance when the VCC is in a UV condi-

tion. The Table below defines the CHRGB LED output

states.

The CHRGB output can be used for a VCC indicator. When

the SC805 is enabled the CHRGB output is either high

or low providing the VCC input is above UVLO.

The CHRGB output signifies the charging status. When

the output current is > Itermination, CHRGB=low. CHRGB

is high when Iout is < Itermination. The CHRGB output is

latched high at the end of the charge cycle when the

output current is less than Itermination. The CHRGB

output will remain high until the timer expires and vbatt

enters a re-charge cycle, or if EN, or VCC are toggled.

Applications Information (Cont.)

The NTC external component configuration is shown in

the typical application schematic on page 1 of the

datasheet. When the NTC voltage from the divider is

above or below the cold and hot temperature threshold

values the SC805 will suspend the charge cycle by turn-

ing off the output, freezing the charge timer, and indicat-

ing a fault on the CHRGB LED. Hysteresis is included for

both hot and cold thresholds to avoid chatter at the NTC

trip points. When the temperature returns to the valid

range the SC805 will automatically resume the charge

cycle. The charge timer will time-out when the SC805

output on-time exceeds the timer setting regardless of

how long it has been disabled due to the NTC tempera-

ture. The internal NTC thresholds of 30% and 75% VCC

were designed to work with Curve-2 type thermistors

available from numerous vendors.

Design Example based on the typical application sche-

matic of page 1 using a Curve 2 Vishay-Dale NTC.

Thot = 50oC

Tcold = 0oC

Find R2 and RT1

Step 1: Pick an RT1 value high enough to prevent self-

heating.

()

V(R T 1 )

RT 1

≥

In general lower values of RT1 provide more noise immu-

nity for the NTC voltage at the expense of bias current

from the input adapter. The dissipation constant DC is

the power rating of the NTC resulting in a 1oC self heat-

ing error.

The DC value for this NTC is 3mW and the voltage across

the NTC is 2.5V for a 5V adapter. Therefore RT1 > 2K,

Set RT1 = 10k

Step 2: Find R2 to set the correct Hot level trip point,

9.52k

2.33

T25

T50

T25

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The closest standard resistor value is 9.53k. RT(temp)/

RT(25oC) is often referred to as the ratio at the given

temp. For this NTC at 50oC the ratio is 0.4086.

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