Rainbow Electronics MAX17127 - Manual (Page 13)

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MAX17127
Six-String WLED Driver with Integrated
Step-Up Converter
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 regu-
lated current sources. Figure 2 shows the MAX17127
functional diagram. When enabled, the step-up control-
ler boosts the output voltage to provide sufficient head-
room for the current sources to regulate their respective
string currents. The MAX17127 features resistor-adjust-
able 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 direct-
ly controlled by the external dimming signal's frequency
and duty cycle.
The MAX17127 has multiple features to protect the con-
troller 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 cur-
rent 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 pro-
tections, 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 128Fs over-
current 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 pro-
vides 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 com-
pared to the internal switch current plus slope com-
pensation 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 lin-
ear 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 approximately
250mV. In standby mode, the internal LDO is in low-power
mode with 10FA (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 1FF 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 detect-
ed 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_ con-
trol. The total startup time is less than 10ms, including
2ms (typ) soft-start. Figure 3 shows the sequence.