AAT3134
High Efficiency 1X/1.5X Fractional
Charge Pump for White LED Applications
remain constant over these address ranges which
Applications Information
provides orthogonal control of the two channels. For
Addresses 1 to 6, D5 and D6 are set to 0mA,
Addresses 7 to 12 are 1mA; Addresses 13 to 18 are
10mA; Addresses 19 to 24 are 12mA; and Addresses
25 to 30 are 15mA. For maximum brightness control,
Addresses 31 and 32 set D5 and D6 to the maximum
20mA level. For design flexibility, D1 to D4 are set to
0mA at Address 31, and 20mA at Address 32. This is
summarized in Table 1 and Figure 1.
Constant Current Output Level Settings
The constant current source output amplitude for out-
put D1 to D4 and D5 to D6 are set via the serial inter-
face according to the scale described in Figure 1 and
the previous section. Because the outputs D1 to D6
are true independent constant current sources, the
voltage observed on any single given output will be
determined by the actual forward voltage (VF) for the
LED being driven.
The modulo 32 interface wraps back to State 1 after
the 32nd clock. With each EN/SET pulse, the output
current changes to the next setting in the address
decoding. To change settings to the previous
address decoding, 31 EN/SET clock pulses are
required. The counter can be clocked at speeds up to
1MHz, so that intermediate states are not visible. The
first rising edge of EN/SET enables the IC and initial-
ly sets the output LED currents to 0mA. Additional
clocks are required to set the desired current level.
Once the final clock cycle is input for the desired
brightness level, the EN/SET pin is held high to main-
tain the device output current at the programmed
level. The device is disabled 500µs after the EN/SET
pin transitions to a logic low state.
Since the output current of the AAT3134 is program-
mable through the S2Cwire serial interface, no PWM
(pulse width modulation) or additional control circuit-
ry are needed to control LED brightness. This feature
greatly reduces the burden on a microcontroller or
system IC to manage LED or display brightness,
allowing the user to "set it and forget it." Furthermore,
with its high-speed serial interface (1MHz data rate),
the output current of the AAT3134 can be changed
successively to brighten or dim LEDs in smooth tran-
sitions (e.g., to fade out) or in abrupt steps, giving the
user complete programmability and real-time control
of LED brightness.
20
18
16
14
12
10
8
6
4
2
0
Address Code
Figure 1: Output Current Level Settings.
8
3134.2005.12.1.1
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
