SC820 datasheet(18/22 Pages) SEMTECH

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SC820

conduction of heat from the die to the ambient environ-

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.

Short Circuit Protection

The SC820 can tolerate a BAT pin short circuit to ground

indefinitely. The current into a ground short (while

BAT

< 1.8V) is approximately 10mA. For V

BAT

> 1.8V, normal

pre-charge current regulation is active.

A short circuit or too little programming resistance to

ground on the IPRGM pin (<< 2.05kΩ) or the IPUSB pin

(<< 4.42kΩ) will prevent proper regulation of the BAT pin

output current for the active programming pin. Prior to

enabling the output a check of the IPRGM and IPUSB pins

is performed to ensure that there is sufficient resistance to

ground. A test current is output on each programming

pin. If the test current produces a voltage of sufficient

amplitude on both programming pins, regardless of input

selected, then the output is enabled. An example with

IPRGM

= 2.94kΩ is illustrated in Figure 5, in which the test

current is applied for approximately 250μs to determine

that there is no pin short. If a short on either program-

ming pin is detected, the test current persists until the

short to ground is removed, and then the charging startup

sequence will continue.

400μs/div

Figure 5 — IPRGM Pin Short-to-Ground Test During

Startup

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

the case that the external VUSB load is removed or is

intermittent.

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

Applications Information (continued)

BAT

(500mA/div)

BAT

=0mA—

IPRGM

=0V—

VIN

(5V/div)

IPRGM

(1V/div)

VIN

=0V—

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