PAM2701 Datasheet, PDF (9/14 Pages) Power Analog Micoelectronics – 4-Channel Regulated Charge Pump White LED Driver

PAM2701
4-Channel Regulated Charge Pump White LED Driver
Application Information
Detailed Description
As shown in the block diagram on page 2, the
main components within the PAM2701 include a
fractional charge pump, mode selection circuit,
output selection logic, LED current setting
detection circuit, and 4 current sense circuits.
The fractional charge pump multiplies the input
voltage a multiple of 1X and 1.5X times the input
voltage. The charge pump switches at a fixed
1MHz when the mode is 1.5X. The charge pump
does not switch during 1X mode, saving power
and improving efficiency.
The mode selection circuit automatically selects
the mode as 1X or 1.5X based on circuit
conditions such as LED voltage, input voltage
and load current. 1X is the more efficient mode
than 1.5X mode.
Table 1 on page 2 shows the output selection
logic control over the LED outputs for on and off
functions with 8 different output states.
The current set and detection circuit uses an
external resistor and a 1.20V reference to
program the LED current.
4 current regulating circuits sink matched
currents from the LEDs. LEDs with matched
forward voltage will produce the best possible
matched currents. For best matching
performance it is recommended that the Vf
between LEDs be under 250mV.
The unused LED channels can be turned off by
CTR0, CTR1 and CTR2, and connecting the
respective LED pins to VOUT pin, in which case,
the corresponding LED driver sink current is only
about 20μA.
Methods for Setting LED Current
There are 4 methods for setting and adjusting the
LED current outlined here. The methods are:
1) R_SET only
2) Analog Reference V_SET
3) PWM Input at CTR0
4) PWM Input at EN
Method 1: LED Current Setting with an
External Resistor R_SET
The most basic means of setting the LED current
is connecting a resistor from R_SET to GND, as
shown in the application circuit on Page 1.
The resistor R_SET establishes the reference
current needed for a constant LED current.
Values of R_SET for a fixed LED current are given in
Table 2, “Typical R_SET Resistance vs. LED
Current”.
Table 2: R_SET Resistance Selection
Standard
I_LED(mA) R_SET(kΩ) _{Value (kΩ)}
Value %
Difference
30
14.7
14.7
0..0%
20
21.8
22.0
0.9%
15
29.3
29.4
0.3%
10
44.1
44.2
0.2%
9
49.4
49.9
1.0%
8
55.7
56.0
0.5%
7
64.2
63.4
0.3%
6
75.0
75.0
0.0%
5
90.0
88.7
-0.3%
4
114.2
115.0
0.7%
3
156.5
158.0
0.9%
2
238.0
237.0
-0.4%
1
540.0
536.0
-0.7%
Method 2: LED Current Setting with a DC
Voltage to RSET Pin
Method 2 is for setting the LED current to control
brightness. An example circuit is shown in Figure
1, employing a 14.7k resistor and an analog input
DC voltage, V , _SET which varies from 1.2V to 0V to
control LED current from 1mA to 30mA. Table 3
shows the resulting output. If necessary, the
analog V_SET voltage can be sourced from a
voltage higher than 1.20V, but the source must
be divided down so that the V_SET mode will not
exceed 1.20V. For lower current and higher
resolution applications, a larger resistor may be
used instead. PWM applications are also
possible with this circuit by application of RC
filtering (Consult with PAM for detail application
support).
Power Analog Microelectronics,Inc
www.poweranalog.com
9
09/2008 Rev 1.3