SP7120AEB Exar Corporation, SP7120AEB Datasheet - Page 4

• Manufacturer Part Number: SP7120AEB
• Description: Display Modules & Development Tools Common Cathode Linear LED Driver
• Manufacturer: Exar Corporation

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
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:
Jan 7 ‘08 RevE
Part Number
SP7120A
SP7120B
SP7120C
SP7122A
SP7122B
SP7122C
SP712x.
The
Current per
channel
15mA
20mA
25mA
15mA
20mA
25mA
Output
bandgap
SP7120/22 Low Dropout LED Driver
Channels
Number
reference
of
2
2
2
3
3
3
page
Channels can be combined for higher output current
Channels can be combined for higher output current
Channels can be combined for higher output current
Channels can be combined for higher output current
Channels can be combined for higher output current
Channels can be combined for higher output current
4
Step 1: Determine the time period of the
PWM frequency
Step 2: take the minimum enable ON time;
this is 15μS for the SP712x parts
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
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
OUTPUT CURRENT SELECTION TABLE
Where PWM
Example
For a 2kHz signal the minimum duty
cycle for the PWM signal is
%Minimum Duty cycle = 3%
value
Comments
thus
CIRCUIT DESCRIPTION
f
is the PWM frequency
increasing
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