Datasheet

Detailed Description
The MAX17127 is a high-efficiency driver for arrays
of white LEDs. It contains a fixed-frequency current-
mode PWM step-up controller, a 5V linear regulator, a
dimming control circuit, an internal power MOSFET,
and six regulated current sources. Figure 2 shows
the MAX17127 functional diagram. When enabled, the
step-up controller boosts the output voltage to provide
sufficient headroom for the current sources to regu-
late their respective string currents. The MAX17127
features resistor-adjustable switching frequency (250kHz
to 1MHz), which allows trade-offs between external
component size and operating efficiency.
The MAX17127 can implement brightness control through
the PWM signal input. The LED current is directly
controlled by the external dimming signal's frequency and
duty cycle.
The MAX17127 has multiple features to protect the
controller from fault conditions. Separate feedback
loops limit the output voltage in all circumstances. The
MAX17127 checks each FB_ voltage during operation.
If one or more strings are open, the corresponding FB_
voltages are pulled below 180mV (max), and an open-
circuit fault is detected. As a result, the respective current
sources are disabled.
When one or more LEDs are shorted and the related
FB_ voltage exceeds 8V, short fault is detected and the
respective current source is disabled if at least one FB_
voltage is lower than the minimum FB_ regulation voltage
+460mV (typ).
When in LED open or short conditions, the fault string is
disabled while other strings can still operate normally.
The MAX17127 also includes other kinds of fault
protections, which are overcurrent, thermal shutdown,
and output overvoltage. The MAX17127 features cycle-
by-cycle current limit to provide consistent operation and
soft-start protection. In an overcurrent condition, the IC
latches off if the fault still exists after a 128µs overcurrent
fault timer expires. The output overvoltage is a nonlatched
operation, and the step-up converter stops switching
during the fault. A thermal-shutdown circuit provides
another level of protection. The MAX17127 is latched off
once thermal shutdown occurs.
The MAX17127 includes a 5V linear regulator that
provides the internal bias and gate driver for the step-up
controller.
Fixed-Frequency Step-Up Controller
The MAX17127’s fixed-frequency, current-mode, step-up
controller automatically chooses the lowest active FB_
voltage to regulate the feedback voltage. Specifically,
the difference between the lowest FB_ voltage and the
current source control signal plus an offset is integrated at
the COMP output. The resulting error signal is compared
to the internal switch current plus slope compensation
to determine the switch on-time. As the load changes,
the error amplifier sources or sinks current to the COMP
output to deliver the required peak inductor current. The
slope-compensation signal is added to the current-sense
signal in order to improve stability at high duty cycles.
Internal 5V Linear Regulator and UVLO
The MAX17127 includes an internal low-dropout linear
regulator (V
DDIO
). When V
IN
is higher than 5.0V, this
linear regulator generates a 5V supply to power the
internal PWM controller, control logic, and MOSFET
driver. The V
DDIO
voltage drops to 3.3V in shutdown. If 5V
< V
IN
< 5.5V, V
DDIO
and V
IN
can be connected together
and powered from an external 5V supply. There is a body
diode from V
DDIO
to V
IN
, so V
IN
must be greater than
V
DDIO
(see Figure 2).
The MAX17127 is disabled until V
DDIO
exceeds the
UVLO threshold. The hysteresis on UVLO is approxi-
mately 250mV. In standby mode, the internal LDO is
in low-power mode with 10µA (max) input current and
approximately regulated at 3.3V (typ). When EN = high,
the internal LDO is enabled and regulated accurately at
5V (typ).
The V
DDIO
pin should be bypassed to AGND with a
minimum 1µF ceramic capacitor.
Startup
At startup, the MAX17127 performs a diagnostic test of
the LED array. In the test phase, all FB_ pins are pulled up
by a given current source (0.4mA min) during 1ms (typ). If
some FB_ voltage is lower than 1.2V (max), the string is
considered to be unused. Therefore, when a string is not
in use, it should be connected to AGND. All other strings
with FB_ higher than 1.2V (max) are detected as in use.
After the LED string diagnostic phases are finished, the
boost converter starts. An additional 1ms after boost
soft-start end is used as minimum FB_ control. The total
startup time is less than 10ms, including 2ms (typ) soft-
start. Figure 3 shows the sequence.
www.maximintegrated.com
Maxim Integrated │ 13
MAX17127 Six-String WLED Driver with
Integrated Step-Up Converter