SP7120CEK-LSLASHTR datasheet(4/17 Pages) SIPEX

4 / 17 page

Jun29-07 RevD

SP7120/22 Low Dropout LED Driver

page

OUTPUT CURRENT SELECTION TABLE

Part Number

Output

Current per

channel

Number

Channels

Comments

SP7120A

15mA

Channels can be combined for higher output current

SP7120B

20mA

Channels can be combined for higher output current

SP7120C

25mA

Channels can be combined for higher output current

SP7122A

15mA

Channels can be combined for higher output current

SP7122B

20mA

Channels can be combined for higher output current

SP7122C

25mA

Channels can be combined for higher output current

CIRCUIT DESCRIPTION

The SP712X drivers are regulated current

sources with an enable input. The SP7120

and SP7122 have factory preset LED

current at three programmed levels: 15mA,

20mA or 25mA (versions A, B, and C

respectively). The SP712X circuit consists

of enable, bandgap reference, and current

amplifier circuit blocks. Enable circuit block

provides the enable and PWM function for

the

SP712x.

The

bandgap

reference

provides a stable voltage source from which

the output current is derived.

ENABLE

The SP712X drivers have a low current

shutdown function. In shutdown mode, the

part draws less than 1µA current maximum.

The part can be set into shutdown mode

using the EN pin (EN = LOW). The enable

pin cannot be left floating. There are no

predefined internal states, so leaving this

pin open will cause the part to operate

incorrectly.

PWM Dimming

The SP712X drivers allow LED-dimming

control by applying PWM signaling to EN

pin. The acceptable frequency range of this

signal is 100Hz to 20 kHz. The minimum

ON time that is required for the Enable pin

is 15µs. Thus for a 2kHz PWM signal, the

acceptable duty cycle range is 3% to 100%.

To find the minimum PWM duty cycle the

following steps are needed:

Step 1: Determine the time period of the

PWM frequency

Where PWM

f is the PWM frequency

Step 2: take the minimum enable ON time;

this is 15µS for the SP712x parts

Example

For a 2kHz signal the minimum duty

cycle for the PWM signal is

%Minimum Duty cycle = 3%

Fault Operation

If one of LEDs is shorted, the voltage across

The Sp712X for that LED pin will be Vcc,

but that channel will still provide the nominal

current

value

thus

increasing

power

dissipation. If all channels are shorted,

excessive power dissipation may damage

the device. If an LED is open, the LED pin

voltage will be pulled up to Vcc, and LED

current will be reduced to 3mA for the other

channels.

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