MAX8822 View Datasheet(PDF) - Maxim Integrated
Part Name
Description
MFG CO.
MAX8822
Maxim Integrated
MAX8822 Datasheet PDF : 15 Pages
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Ultra-Efficient Negative Charge-Pump LED
Driver with Dual LDOs in 3mm x 3mm Thin QFN
Table 3. ENLDO Pulse Count and LDO1
and LDO2 Output Voltage Selection
ENLDO PULSE
COUNT*
PROGRAMMED
LDO1 VOLTAGE
(V)
PROGRAMMED
LDO2 VOLTAGE
(V)
0**
0
0
1
2.8
1.2
2
3.3
1.2
3
2.8
1.5
4
3.3
1.5
5
1.2
1.8
6
1.8
1.8
7
2.8
1.8
8
3.3
1.8
9
2.8
2.8
10
0
1.5
11
0
1.8
12
0
2.8
13
1.8
0
14
2.8
0
15***
3.3
0
*Rising edge.
**Initial pulse is only required to enable LDOs from shutdown.
***If ENLDO is pulsed for more than 15 counts,VLDO1 and
VLDO2 are 0V (not shutdown) when the count is latched (the
count does not roll over).
Shutdown Mode
The MAX8822 features a shutdown mode to reduce
power consumption. Hold ENLED low for at least 2.5ms
to place the LEDs in shutdown. LED shutdown pulls
NEG to GND through a 10kΩ internal resistor. Hold
ENLDO low for at least 2.5ms to place the LDOs in
shutdown. LDO shutdown pulls LDO_ to GND through
a 1kΩ internal resistor. The MAX8822 consumes only
0.5µA of supply current when both the LED current reg-
ulators and LDOs are in shutdown mode.
LDO and LED Thermal Shutdown
The MAX8822 includes two thermal-limit circuits that
protect the IC from temperatures exceeding +160°C
(typ). The first circuit monitors the internal LED_ regula-
tor temperature and shuts down the entire IC when the
temperature limit is violated. The second circuit moni-
tors the internal LDO temperature and shuts down only
the LDOs when the temperature limit is violated. The
thermal-shutdown condition clears after temperature
has cooled by 20°C (typ) and resets the LEDs and
LDOs in the default condition of ILED_= 24mA and
VLDO1 = VLDO2 = 0V.
Power-Up LED Short Detection
and Open-Fault Protection
The MAX8822 contains special circuitry to detect an
LED_ short-circuit condition at startup only, and dis-
ables that current regulator to avoid wasting battery
power. If an LED fails short-circuit detection after start-
up, that current regulator continues current-regulated
operation until IC power is cycled and the short circuit
is detected during the subsequent startup.
An open-circuit LED failure drives the voltage on the
corresponding LED current-regulator output below the
switchover threshold, enabling the charge pump.
Applications Information
Input Ripple
For LED drivers, input ripple is more important than out-
put ripple. The amount of input ripple depends on the
source supply’s output impedance. Add a lowpass filter
to the input of the MAX8822 to further reduce input rip-
ple. Alternatively, increasing CIN reduces input ripple.
Using Fewer Than Four LEDs
The MAX8822 can operate with fewer than four LEDs.
Disable the unused current regulator by connecting
LED_ to IN. If an unused LED_ is not connected to IN,
the MAX8822 operates as if an open LED has been
detected. In this condition, the open-circuit LED_ drives
the voltage on the corresponding output below the
switchover threshold, enabling the charge pump.
Component Selection
Ceramic capacitors are recommended due to their small
size, low cost, and low ESR. Select ceramic capacitors
that maintain their capacitance over temperature and DC
bias. Capacitors with X5R or X7R temperature character-
istics generally perform well. Recommended values are
shown in the Typical Operating Circuit. Using a larger
value input capacitor helps to reduce input ripple (see
the Input Ripple section).
The LDO output capacitor size affects LDO stability. A
1µF ceramic capacitor is recommended to maintain
stability for load currents up to 200mA.
PCB Layout and Routing
The MAX8822 is a high-frequency switched-capacitor
voltage regulator. For best circuit performance, use a
solid ground plane and place all capacitors as close
as possible to the IC. Use large traces for the power-
supply inputs to minimize losses due to parasitic trace
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