SC813 datasheet(19/23 Pages) SEMTECH

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SC811 / SC813

Applications Information (continued)

straint (along with CC, CV, and CT constraints), but it is

active only when one of the USB modes is selected.

In either of the USB modes, if the VIN voltage is externally

pulled below V

UVLR

, the UVLR feature will reduce the charg-

ing current to zero. This condition will not be interpreted

as termination and will not result in an end-of-charge indi-

cation. The STATB pin will remain asserted as if charging is

continuing. This behavior prevents repetitive indications

of end-of-charge alternating with start-of-charge in the

case that the external VIN load is removed or is

intermittent.

USB High Power and Low Power Support

The USB specification restricts the load on the USB Vbus

power network to 100mA for low power devices and for

high power devices prior to granting permission for high

power operation. The specification restricts the Vbus load

to 500mA for high power devices after granting permis-

sion to operate as a high power device. A fixed 1:5 ratio of

low power to high power charging current is desirable for

charging batteries with maximum fast-charge current of

at least 500mA. For this application, the SC811/3 provides

fixed 1:5 current ratio low-to-high power mode support,

via the tri-level MODE input pin.

For batteries with maximum fast-charge current less than

500mA, a fixed 1:5 low/high power charge current ratio

will result in suboptimal charging in USB low power mode.

For example, a 250mAh battery will typically require a

fast-charge current of 250mA or less. A fixed 1:5 ratio for

USB low-to-high power charging current will unnecessar-

ily reduce charging current to 50mA, well below the

100mA permitted. In this case, it may be preferable to

program USB low-power fast-charge current by switching

an external programming resistor. See the section Design

Considerations — Small Battery.

Input Over-Voltage Protection

The VIN pin is protected from over-voltage to at least 30V

above GND. When the input voltage exceeds the Over-

Voltage Protection (OVP) rising threshold ( VT

OVP-R

charging is halted. When the input voltage falls below the

OVP falling threshold (VT

OVP-F

), charging restarts. An OVP

fault turns off the STATB output. STATB is turned on again

when charging restarts.

The OVP threshold of the SC811 has been set relatively

high to permit the use of poorly regulated adapters. Such

adapters may output a high voltage until loaded by the

charger. A too-low OVP threshold could prevent the

charger from ever turning on and loading the adapter to

a lower voltage. If the adapter voltage remains high

despite the charging load, the fast thermal limiting feature

will immediately reduce the charging current to prevent

overheating of the SC811. This behavior is illustrated in

Figure 4, in which V

BAT

= 3.0V, I

= 700mA, and V

VIN

stepped from 0V to 8.1V. Initially, power dissipation in the

SC811 is 3.6W.

1s/div

VIN

(2V/div)

VIN

BAT

=0V—

BAT

(100mA/div)

BAT

=0mA—

VIN

=8.1V, V

BAT

=3.0V

BAT

=700mA (Initially), P

DISSIPATION

=3.6W (Initially)

BAT

(2V/div)

Figure 4 — SC811 Thermal Limiting Example

Notice the BAT output current is rapidly reduced to limit

the internal die temperature, then continues to decline as

the circuit board gradually heats up, further reducing the

conduction of heat from the die to the ambient environ-

ment. The fast thermal limiting feature ensures compliance

with CCSA YD/T 1591-2006, Telecommunication Industrial

Standard of the People’s Republic of China — Technical

Requirements and Test Method of Charger and Interface for

Mobile Telecommunication Terminal, Section 4.2.3.1.

Alternatively, the SC813 is offered for users who want to

limit OVP to a guaranteed maximum of 6V. The SC811 and

SC813 are alike except for OVP threshold.

Short Circuit Protection

The SC811/3 can tolerate a BAT pin short circuit to ground

indefinitely. The current into a ground short is approxi-

mately 10mA.

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