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MAX8834Y/MAX8834Z
Adaptive Step-Up Converters
with 1.5A Flash Driver
Detailed Description
The MAX8834Y/MAX8834Z flash drivers integrate an
adaptive 1.5A PWM step-up DC-DC converter, two
750mA white LED camera flash/movie current regula-
tors, and a 16mA indicator LED current regulator. An
I 2 C interface controls individual output on/off, the step-
up output voltage setting, the movie/flash current, and
the flash timer duration settings.
Step-Up Converter (LX, OUT, COMP, PGND)
The MAX8834Y/MAX8834Z include a fixed-frequency,
PWM step-up converter that supplies power to the flash
LEDs. The output voltage is programmable from 3.7V to
5.2V (in 100mV steps) through the I 2 C interface. The
output voltage can also be set adaptively based on the
LED forward voltage. The step-up converter switches
an internal power MOSFET and synchronous rectifier at
a constant 2MHz or 4MHz frequency, with varying duty
cycle up to 75%, to maintain constant output voltage as
the input voltage and load vary. Internal circuitry pre-
vents any unwanted subharmonic switching by forcing
a minimum 7% (typ) duty cycle.
When the step-up converter is set to dropout mode, the
internal synchronous rectifier is driven fully on, keeping
the voltage at OUT equal to the LX input. This mode
provides the lowest current consumption when driving
LEDs with low forward voltage.
The output voltage is internally monitored for a fault
condition. If the output voltage drops below 8% (typ) of
the nominal programmed value, a POK fault is indicat-
ed in STATUS1 register bit 5. This feature is disabled if
the step-up converter is set to operate in adaptive
mode.
Overvoltage Protection
The MAX8834Y/MAX8834Z include a comparator to
monitor the output voltage (V OUT ) during adaptive
mode operation of the step-up converter. If at anytime
the output voltage exceeds a maximum threshold of
5.5V, the COMP capacitor is discharged until the output
voltage is reduced by the 200mV (typ) hysteresis. Once
the output voltage drops below this threshold, normal
charging of the COMP capacitor is resumed.
Flash Current Regulator
(FLED1 and FLED2)
A low-dropout linear current regulator from FLED1/
FLED2 to FGND sinks current from the cathode terminal
of the flash LED(s). The FLED1/FLED2 current is regu-
lated to I 2 C programmable levels for movie mode (up to
125mA, see Table 5) and flash mode (up to 750mA,
12
see Tables 3 and 4). The movie mode provides continu-
ous lighting when enabled through I 2 C or LED_EN.
When the flash mode is enabled, a flash safety timer,
programmable from 50ms to 800ms through I 2 C, limits
the duration of the flash mode. Once the flash safety
timer expires, the current regulators return to movie
mode if movie mode was active when a flash event was
triggered. The flash mode has priority over the movie
mode.
Flash Safety Timer
The flash safety timer is activated any time flash mode is
selected, either with LED_EN or through the I 2 C interface.
The flash safety timer, programmable from 50ms to
800ms through I 2 C, limits the duration of the flash mode
in case LED_EN is stuck high or the I 2 C command to
turn off has not been sent within the programmed flash
safety timer duration. This timer can be configured to
operate either in one-shot mode or maximum flash
duration mode (see Table 9). In one-shot mode, the
flash function is initiated on the rising edge of LED_EN
(or I 2 C bit) and terminated based on the programmed
value of the safety timer (see Figure 1). In the maximum
flash timer mode, flash function remains enabled as
long as LED_EN (or I 2 C bit) is high, unless the prepro-
grammed safety timer times out (see Figure 2).
Once the flash mode is disabled, by either LED_EN,
I 2 C, or flash safety timer, the flash has to be off for a
minimum time (flash safety timer reset inhibit period),
before it can be reinitiated (see Figure 3). This prevents
spurious events from re-enabling the flash mode.
Indicator Current Regulator (INDLED)
A low-dropout linear current regulator from INDLED to
FGND sinks current from the cathode terminal of the
indicator LED. The INDLED current is regulated to I 2 C
programmable levels up to 16mA. Programmable con-
trol is provided for ramp-up (OFF to ON) and ramp-
down (ON to OFF) times, as well as blink rate and duty
cycle. The user can choose to enable or disable the
ramp time and blink rate features. See Tables 6, 7, and
8 for more information.
INDLED Blink Function
INDLED current regulator is able to generate a blink
function. The OFF and ON time for INDLED are set
using the I 2 C interface. See Figure 4.
INDLED Ramp Function
The INDLED current regulator output provides ramp-up/
down for smooth transition between different brightness
settings. The ramp-up/down times are controlled by the
Maxim Integrated
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