SC820 datasheet(18/21 Pages) SEMTECH

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SC820

ment. In this experiment, the final steady-state BAT

current was 462mA at T

= 25C on the SC820 evaluation

board. The fast thermal limiting feature ensures compli-

ance 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.

Operation Without a Battery

The SC820 can be operated as a 4.2V LDO regulator

without the battery present, for example, factory testing.

If this use is anticipated, the output capacitance C

BAT

should be at least 2.2μF to ensure stability. To operate the

charger without a battery, the ENB pin must be driven low

or grounded.

Capacitor Selection

Low cost, low ESR ceramic capacitors such as the X5R and

X7R dielectric material types are recommended. The BAT

pin capacitor range is 1μF to 22μF. The VAD pin and VUSB

input capacitors are typically between 0.1μF and 2.2μF,

although larger values will not degrade performance.

Capacitance must be evaluated at the expected bias

voltage, rather than the zero-volt capacitance rating.

PCB Layout Considerations

Layout for linear devices is not as critical as for a switching

regulator. However, careful attention to detail will ensure

reliable operation.

Place input and output capacitors close to the

device for optimal transient response and device

behavior.

Connect all ground connections directly to the

ground plane. If there is no ground plane,

connect to a common local ground point before

connecting to board ground near the GND pin.

Attaching the part to a larger copper footprint

will enable better heat transfer from the device,

especially on PCBs with internal ground and

power planes.

Design Considerations — USB Charging

The USB specification restricts the load on the USB Vbus

power network to 100mA for low power devices and for

•

programmed by IPRGM or IPUSB, determined by input

selection), depending on the voltage at the output.

Input Over-Voltage Protection

The VAD and VUSB input pins are protected from over-

voltage to at least 30V above GND. When the voltage of

the selected input 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 resumes. Note that the VAD input

remains selected even in the case that the VAD voltage

exceeds the OVP threshold. An excessive VAD voltage will

disable charging despite the presence of a valid VUSB

voltage. An OVP fault turns off the STATB output. STATB is

turned on again when charging restarts.

The OVP threshold 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

SC820. This behavior is illustrated in Figure 4, in which V

BAT

= 3.0V, I

= 700mA, and V

VAD

is stepped from 0V to 8.1V.

Initially, power dissipation in the SC820 is 3.6W.

1s/div

VAD

(2V/div)

VAD

BAT

=0V—

BAT

(100mA/div)

BAT

=0mA—

VAD

=8.1V, V

BAT

=3.0V

BAT

=700mA (Initially), P

DISSIPATION

=3.6W (Initially)

BAT

(2V/div)

Figure 4 — 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-

Applications Information (continued)

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