SC805AMLTRT datasheet(10/17 Pages) SEMTECH

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 2007 Semtech Corp.

SC805A

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POWER MANAGEMENT

Applications Information (Cont.)

due to the NTC temperature. 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 schematic

of Page using a Curve--2, Vishay-Dale NTC.

Thot = 50°C

Tcold = 0°C

Find R2 and RT

Step : Pick an RT value high enough to prevent self-

heating.

V(RT

In general, lower values of RT 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 °C self-heating error.

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

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

Set RT = 0k

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

9.52k

2.33

T25

T50

T25

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

RT(25°C) is often referred to as the ratio at the given temp.

For this NTC at 50°C, the ratio is 0.4086.

Step 3: Calculate the Tcold value.

2.86

10K

COLD

The ratio for the Curve-2 NTC at 0°C is 2.86. The differ-

ence represents less than /2°C (i.e., actual trip would be

-0.5°C instead of 0°C.) For temperature trip values other

than 0°C and 50°C, alternate curve-type NTC elements can

be used or an additional resistor can be added to the Rntc

divider to alter the temperature coefficient.

CHRGB LED Flag

The CHRGB LED driver on the SC805A 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 condition. The

table below defines the CHRGB LED output states.

LED

High

Low

High-Z

Blinking

CHRGB

Iout <

Iterm

Iout > term VCC < UV

Pre-charge

OV or NTC

Fault

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

the SC805A 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 greater than termination current,

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. The CHRGB indicator operates the same way in

both charge and LDO mode.

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

the SC805A is enabled the CHRGB output is either high or

low providing the VCC input is above UVLO. Alternately an

LED connected to VCCIL can be used for a VCC indicator as

shown in the typical application schematic on Page .

There are five fault modes detected by the SC805A; (input

under-voltage, input over-voltage, NTC temperature, max

die temperature (or OT) and pre-charge timeout). The LED

states for these fault modes are shown below.

Fault

CHRGB

VCC UVLO

High-Z

VCC OVP

Blinking

NTC Fault

Blinking

Pre-Charge Time-Out

Blinking

J > 150°C

Blinking

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