SC614EVB datasheet(13/25 Pages) SEMTECH

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POWER MANAGEMENT

SC614

United States Patent No. 6,504,422

www.semtech.com

POWER MANAGEMENT

would be between 18.4mA and 21.6mA (+/-8%). All 7

outputs meet this requirement over the industrial

temperature range. To calculate the accuracy based

upon the actual measured LED current, I

LED_ACC, use the

following formula:

(

)

100

)

SET

(

LED

)

SET

(

LED

)

Measured

(

LED

ACC

LED

•

−

Current Matching

The current matching percentage is a figure that

represents how closely matched LEDs are that are set

to the same current. For any particular LED within a group

of LEDs set to the same current, the matching is the

ΔI

for that LED from the average of the minimum and

maximum value of the group (i.e. the center of the

measured current range) expressed as a percentage of

that average.

Current matching is calculated as follows:

100

MIN

MAX

MIN

MAX

LED

•

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

•

⎟⎟

⎟

⎠

⎞

⎜⎜

⎜

⎝

⎛

⎟

⎠

⎞

⎜

⎝

⎛

Which can be reduced to:

(

)

()

100

MIN

MAX

MIN

MAX

LED

•

−

Protection Circuitry

The SC614 also provides protection circuitry that

prevents the device from operating in an unspecified

state. These include Output Over-Voltage Protection

(OVP), Over-Temperature Protection (OTP), Over-Current

Protection (OCP) and Short-Circuit Protection (SCP).

Output Over-Voltage Protection

Output over-voltage protection is included to prevent the

SC614 from generating an output voltage that could

damage other devices connected to it such as load LEDs

any bypass capacitors. When the output voltage exceeds

5.7V, the OVP circuitry disables the charge pump until

the output voltage decreases to an acceptable level.

Usually the only reason for the output voltage to trip OVP

is if one of the LEDs goes open. If this happens the

SC614 will raise the output voltage to attempt to bring

Applications Information (Cont.)

that LED current back into regulation. When the OVP trip

point is reached, the charge pump will be turned off. Any

current sinks that measure close to ground will be turned

off in an attempt to isolate the faulty LED. Once the

output voltage drops enough the charge pump will

resume operation.

Over-Temperature Protection

The over-temperature protection circuitry helps to

prevent the device from overheating and experiencing a

catastrophic failure. When the junction temperature

exceeds 160°C the output is disabled and the device

enters sleep mode. All register settings are retained. The

junction temperature must drop by more than the

hysteresis of 10°C before the part exits sleep mode and

re-commences normal operation.

Over-Current and Short Circuit Protection

Adaptive current limit circuitry is provided to protect the

device from various levels of shorts from resistive to full

shorts as well as to limit in-rush current at start-up and

during mode transitions. The current limit levels adjust

to the total output current set for the LEDs and thus will

be higher when very high levels of currents are

programmed, such as for flash operation. The current

limit levels are set to ensure that the device will not

current limit under normal operation. When an output

short circuit occurs, the device folds back the current

limit level to a nominal 300mA. If sustained current limit

occurs the device may shut down due to internal heating

triggering the OTP circuitry.

Capacitor Selection

The SC614 is designed to use low-ESR ceramic

capacitors for all four external capacitors: input, output

and charge pump bucket capacitors. Ideal performance

is achieved when the bucket capacitors (C3 and C4 in

the application circuit) are exactly equal. Note: It is

recommended that X5R or X7R capacitors are used for best

performance.

Thermal Resistance and Heat Management

The SC614 is packaged in a thermally efficient MLPQ24

package that has a thermal pad to remove the heat from

the part. It is intended to be connected using multiple

vias to the ground plane, and the thermal resistance

rating of 40°C/W reflects this. A good layout will enable

the part to operate at maximum output current ratings

without tripping the OTP circuitry.

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