AAT3340_08 Datasheet, PDF (9/14 Pages) Advanced Analogic Technologies – High Efficiency 1x/1.5x Charge Pump

ChargePump^TM
S²Cwire
Data
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
LED Current
(mA)
20.0
19.4
18.7
18.1
17.4
16.8
16.1
15.5
14.8
14.2
13.6
12.9
12.3
11.6
11.0
10.3
S²Cwire
Data
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
LED Current
(mA)
9.7
9.0
8.4
7.7
7.1
6.5
5.8
5.2
4.5
3.9
3.2
2.6
1.9
1.3
0.6
0.3
Table 2: S²C Data vs LED Current.
EN/SET Serial Interface
The LED current magnitude is controlled by the EN/SET
pin using the S²Cwire interface. The interface records
rising edges of the EN/SET pin and decodes them into 32
individual current level settings. Code 1 is full scale, and
Code 32 is 0.3mA. The modulo 32 interface wraps states
back to state 1 after the 32nd clock. The counter can be
clocked at speeds up to 1MHz, so intermediate states are
not visible. The first rising edge of EN/SET enables the
IC and initially sets the output LED current to full scale,
the lowest setting equal to 0.3mA. Once the final clock
cycle is input for the desired brightness level, the EN/
SET pin should be held high to maintain the device out-
PRODUCT DATASHEET
AAT3340
High Efficiency 1x/1.5x Charge Pump
put current at the programmed level. The device is dis-
abled 500μs after the EN/SET pin enters a logic low
state. The EN/SET timing is designed to accommodate a
wide range of data rates. After the first rising edge of
EN/SET, the charge pump is enabled and reaches full
capacity after the soft-start time (T_SS). Exact counts of
clock pulses for the desired current level should be
entered on the EN/SET pin with a single burst of clocks.
The counter refreshes each time a new clock input to the
EN/SET pin is detected. A constant current is sunk as
long as EN/SET remains in a logic high state. The current
sink pins are switched off after EN/SET has remained low
state for at least the t_OFF timeout period (see Figure 1).
LED Selection
The AAT3340 is designed to drive white LEDs with for-
ward voltages up to 4.8V. Since BL1 through BL4 output
current sinks are matched with negligible voltage depen-
dence; the LED brightness will be matched regardless of
their forward voltage matching.
Device Switching Noise Performance
The AAT3340 operates at a fixed frequency of 1MHz to
control noise and limit harmonics that can interfere with
the RF operation of cellular telephone handsets or other
communication devices. Back-injected noise appearing
on the input pin of the charge pump is 20mV peak-to-
peak, typically ten times less than inductor-based DC/DC
boost converter white LED backlight solutions. The
AAT3340 soft-start feature prevents noise transient
effects associated with inrush currents during startup of
the charge pump circuit.
tHI
tLO
tOFF
OFF
1
2
3
Figure 2: EN/SET Timing Diagram.
OFF
3340.2008.07.1.1
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