Rainbow Electronics MAX8645Y - Manual (Page 10)

MAX8645X/MAX8645Y
1x/1.5x/2x White LED Charge Pumps with Two
LDOs in 4mm x 4mm TQFN
10 ______________________________________________________________________________________
Thermal Shutdown
The MAX8645X/MAX8645Y include a thermal-limit circuit
that shuts down the IC at approximately +160°C. Turn-
on occurs after the IC cools by approximately 20°C.
Applications Information
Setting the Main Output Current
SETM controls M1–M6 regulation current. Current flow-
ing into M1, M2, M3, M4, M5, and M6 is a multiple of
the current flowing out of SETM:
I M1 = I M2 = I M3 = I M4 = I M5 = I M6 = K x (0.6V / R SETM )
where K = 23, 69, or 230 (depending upon the state of
ENM1 and ENM2; see Table 1), and R
SETM is the resis-
tor connected between SETM and GND (see the
Typical Operating Circuit).
Setting the Flash Output Current
SETF controls the F1, F2 regulation current. Current
flowing into F1 and F2 is a multiple of the current flow-
ing out of SETF:
I F1 = I F2 = N x (0.6V / R SETF )
where N = 1380.
Single-Wire Pulse Dimming
For more dimming flexibility or to reduce the number of
control traces, the MAX8645X/MAX8645Y support serial
pulse dimming. Connect ENM1 and ENM2 together to
enable single-wire pulse dimming of the main LEDs (or
ENF only for single-wire pulse dimming of the flash
LEDs). When ENM1 and ENM2 (or ENF) go high
simultaneously, the main (or flash) LEDs are enabled at
full brightness. Each subsequent low-going pulse
(500ns to 250µs pulse width) reduces the LED current
by 3.125% (1/32), so after one pulse, the LED current is
96.9% (or 31/32) x I
LED . The 31st pulse reduces the
current to 0.03125 x I LED . The 32nd pulse sets the LED
current back to I LED . Figure 1 shows a timing diagram
for single-wire pulse dimming. Because soft-start is
longer than the initial t
HI , apply dimming pulses quickly
upon startup (after initial t HI ) to avoid LED current tran-
sitioning through full brightness.
Simple On/Off Control
If dimming control is not required, connect ENM1 to
ENM2 for simple on/off control. Drive both ENM1 and
ENM2 to a logic-level high to turn on the main LEDs.
Drive both ENM1 and ENM2 to a logic-level low to turn
off the main LEDs. ENF is the simple on/off control for
the flash LEDs. Drive ENF to a logic-level high to turn
on the flash LEDs. Drive ENF to a logic-level low to turn
off the flash LEDs. In this case, LED current is set by
the values of R
SETM and R SETF .
Dimming Using PWM into ENM1
Use ENM2 for shutdown and drive ENM1 with a PWM
signal. LED brightness can be varied from 10% to full
brightness based upon the duty cycle of the PWM sig-
nal. Drive ENM2 high to keep the IC on, eliminating any
soft-start delay that would impede PWM control and
allowing a PWM frequency up to 5kHz (Figure 2).
Driving Fewer than Eight LEDs
When driving fewer than eight LEDs, two connection
schemes can be used. The first scheme is shown in
Figure 3 where LED drivers are connected together.
This method allows increased current through the LED
and effectively allows total LED current to be I
LED multi-
plied by the number of connected drivers. The second
method of connection is shown in Figure 4 where stan-
dard white LEDs are used and fewer than eight are
connected. This scheme does not alter current through
each LED but ensures that the unused LED driver is
properly disabled.
1 0 2 4 3 5 27 28 29 30 31 32
SHUTDOWN
t SHDN
2ms (typ)
2/32 1/32
3/32
4/32
5/32
27/32
28/32
29/32
30/32
31/32 31/32
32/32 32/32
SHUTDOWN
I M_ OR I F_
ENM1 AND ENM2
OR
ENF
t SOFT-START
INITIAL t HI
t LO t HI
≥ 200μs
≥ 500ns 500ns TO 250 μs
Figure 1. ENM_ and ENF Timing Diagram
ENM1/ENM2 STATES BRIGHTNESS
M1–M6
CURRENT
ENM1 = low, ENM2 = low Shutdown 0
ENM1 = low, ENM2 = high 1/10 brightness 23 x I SETM
ENM1 = high, ENM2 = low 3/10 brightness 69 x I SETM
ENM1 = high, ENM2 = high Full brightness 230 x I SETM
Table 1. ENM1/ENM2 States