MAX8630W/MAX8630X
125mA 1x/1.5x Charge Pumps for 5 White LEDs
in 3mm x 3mm TDFN
8 _______________________________________________________________________________________
Detailed Description
The MAX8630_ charge pump drives up to 5 white LEDs
(3 main LEDs and 2 sub LEDs) with regulated constant
current for uniform intensity. By utilizing adaptive 1x/1.5x
charge-pump modes and very low-dropout current regu-
lators, it achieves 125mA guaranteed output-drive capa-
bility and high efficiency over the 1-cell Li+ battery input
voltage range. 1MHz fixed-frequency switching allows
for tiny external components and the regulation scheme
is optimized to ensure low EMI and low input ripple. The
MAX8630X provides independent on/off/dimming control
for main and sub displays (see Figure 1). The
MAX8630W allows direct-PWM dimming of all five LEDs
together (see Figure 2). A functional diagram for the
MAX8630X/MAX8630W is shown in Figure 3.
1x to 1.5x Switchover
When V
IN
is higher than V
OUT
, the MAX8630_ operates
in 1x mode and V
OUT
is pulled up to V
IN
. The internal
current regulators regulate the LED current. As V
IN
drops, V
LEDMIN_
eventually falls below the switchover
threshold of 88mV, and the MAX8630_ starts switching
in 1.5x mode, and V
LEDMIN
is regulated to 122mV by
the charge pump. To switch back to 1x mode, the
MAX8630_ determines if V
IN
- V
OUT
is sufficient to keep
V
LEDMIN
greater than 88mV. The comparator that
makes this judgment is adaptive and matches the
switchover for the conditions.
Soft-Start
The MAX8630_ includes soft-start circuitry to eliminate
inrush current at turn-on. When starting up, the output
capacitor is charged directly from the input with a
ramped current source (with no charge-pump action)
until the output voltage approaches the input voltage.
Once this occurs, the charge pump enters 1x mode,
and the LED output current is then ramped up in 32 lin-
ear steps. If the current regulators are in dropout at the
end of this time, the charge pump switches to 1.5x
mode. If the output is shorted to ground (V
OUT
<
1.25V), the part stays in the initial soft-start stage and
output current is limited by the ramped current source.
Additionally, when the main or sub LED current rolls
over from 1/32 to full, the LED current regulators soft-
start again to eliminate input current spikes.
ENM/ENS Dimming Control (MAX8630X)
When the LEDs are enabled (by driving EN_ high), the
MAX8630X goes through soft-start and brings the LED
current up in 32 linear steps. Dim the MAX8630X by puls-
ing EN_ low (500ns to 500µs pulse width). Each pulse
reduces the LED current by 1/32nd. After 31 pulses, the
current reaches 1/32, and the next pulse restores the cur-
rent to 100%. Figure 4 shows a timing diagram for EN_.
If dimming control is not required, ENM and ENS work
as simple on/off controls. Drive ENM/ENS high to
enable the LEDs, or drive ENM/ENS low for shutdown.
The LEDs operate at 100% brightness under these sim-
ple on/off conditions.
PWM Dimming Control (MAX8630W)
Dim the MAX8630W by applying a direct-PWM logic-
level signal to PWM. An internal resistor combined with
the capacitor at CPWM forms a lowpass filter that con-
verts the PWM signal to DC LED current that is propor-
tional to the PWM signal’s duty cycle. All five LEDS are
controlled together on the MAX8630W. The PWM fre-
quency range is 900Hz to 200kHz.
If dimming control is not required, PWM works as a sim-
ple on/off control. Drive PWM high to enable the LEDs,
or drive PWM low for shutdown.
