WLED brightness is controlled by turning the WLEDs on
and off with a DPWM signal. The DPWM frequency can
be accurately adjusted with a resistor or directly controlled
by the PWMI signal. The brightness of the LEDs is pro-
portional to the duty cycle of the DPWM signal, which
is controlled externally through either a PWM or 2-wire
SMBus-compatible interface, or both. When both inter-
faces are used at the same time, the product of the PWM
duty cycle and SMBus command value is used for the
dimming control. This DPWM control provides a dimming
range with 8-bit resolution.
The MAX17105 has multiple features to protect the
controller from fault conditions. Separate feedback
loops limit the output voltage in all circumstances. The
MAX17105 checks each FB_ voltage during the opera-
tion. If one or more strings are open, the corresponding
FB_ voltages are pulled below 220mV (typ), 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 FB_ voltage exceeds 8V, a short
fault is detected and the respective current source is
disabled. In either LED open or short conditions, the fault
strings are disabled while other strings can still operate
normally. The controller features cycle-by-cycle current
limit to provide consistent operation and soft-start pro-
tection. In a current-limit condition, the controller shuts
down after a 128µs overcurrent fault timer expires. A
thermal-shutdown circuit provides yet another level of
protection.
The MAX17105 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 MAX17105’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 (VSAT) is
integrated at the COMP output. The resulting error signal
is compared to the external 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 MAX17105 includes an internal low-dropout (LDO)
linear regulator (VCC). When VIN is higher than 5.5V,
this linear regulator generates a 5V supply to power
the internal PWM controller, control logic, and MOSFET
driver. The VCC voltage drops to 4.5V in standby. If VIN is
less than or equal to 5.5V, VCC and IN can be connected
together and powered from an external 5V supply. There
is a body diode from VCC to IN, so VIN must be greater
than VCC. (See Figure 2.)
The MAX17105 includes power-on reset (POR) and
undervoltage lockout (UVLO) features. POR resets the
fault latch and sets all the SMBus resisters to their POR
values. POR occurs when VCC rises above 2.8V (typ).
The controller is disabled until VCC exceeds the UVLO
threshold of 4.3V (typ). The hysteresis on VCC UVLO is
approximately 85mV.
In standby mode, the internal LDO is low-power mode
with 60µA (max) input current and regulated at 4.5V (typ).
When EN is high in direct-PWM mode or an ENABLE
command through the SMBus interface, the internal LDO
is enabled and regulated at 5.0V (typ). In addition to VCC
UVLO, the MAX17105 has also implemented an IN UVLO
function. This IN UVLO function is not activated in direct-
PWM mode. However, when the controller is enabled with
SMBus, the IN UVLO feature is enabled. If the IN voltage
is below its UVLO threshold, the step-up converter is shut
off.
The VCC pin should be bypassed to SGND with a mini-
mum 1µF ceramic capacitor.
Startup
The MAX17105 has two operating modes: direct-PWM
mode and SMBus mode. The two modes are identified
by the DFSET pin. At the first enable signal, either EN =
high or an SMBus ENABLE command, the DFSET pin
sources a current to check whether the DFSET pin is
connected to SGND. When the DFSET voltage is lower
than 210mV (typ), the MAX17105 is set for direct-PWM
mode. Otherwise, the MAX17105 is set for SMBus mode.
Once the mode is set, the mode is not cleared until the
VCC is lower than the POR voltage. When the MAX17105
is set for SMBus mode, the EN pin signal is ignored
and the MAX17105 is woken by the SMBus ENABLE
command. When the MAX17105 is set for direct-
PWM mode, the SMBus command is ignored and the
MAX17105is woken by the EN signal. Table 3 summa-
rizes the operating mode.
www.maximintegrated.com Maxim Integrated
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15
MAX17105 8-String WLED Driver with Integrated Step-Up
Regulator and SMBus/PWM Dimming Capability