Frequency Selection
A tri-level OSC input sets the internal oscillator frequen-
cy for step-up converter, as shown in Table 1. High-fre-
quency (1MHz) operation optimizes the regulator for
the smallest component size, at the expense of efficien-
cy due to increased switching losses. Low-frequency
(500kHz) operation offers the best overall efficiency but
requires larger components and PCB area.
Overvoltage Protection
To protect the step-up converter when the load is open,
or the output voltage becomes excessive for any rea-
son, the MAX17061 features a dedicated overvoltage
feedback input (OV). The OV pin is tied to the center
tap of a resistive voltage-divider from the high-voltage
output. When the OV pin voltage, V
OV
, exceeds
1.236V, a comparator turns off the internal power MOS-
FET. This step-up converter switch is reenabled after
the V
OV
drops 60mV (typ) hysteresis below the protec-
tion threshold. This overvoltage-protection feature
ensures the step-up converter fail-safe operation when
the LED strings are disconnected from the output.
LED Current Sources
Maintaining uniform LED brightness and dimming capa-
bility are critical for LCD backlight applications. The
MAX17061 is equipped with a bank of eight matched
current sources. These specialized current sources are
accurate to within ±1.5% between strings and can be
switched on and off within 15µs, enabling PWM fre-
quencies of up to 2kHz. All LED full-scale currents are
identical and are set through the ISET pin (15mA < I
LED
< 30mA). When ISET is connected to V
CC
, the LED full-
scale current is set at the 25mA default value.
The minimum voltage drop across each current source
is approximately 625mV when the LED current is 25mA.
The low-voltage drop helps reduce dissipation while
maintaining sufficient compliance to control the LED
current within the required tolerances.
The LED current sources can be disabled by tying the
respective FB_ pin to V
CC
at startup. When the IC is
powered up, the controller scans settings for all FB_
pins. If a FB_ pin is not tied to V
CC
, an internal circuit
pulls this pin low, and the controller enables the corre-
sponding current source to regulate the string current.
If the FB_ pin is tied to V
CC
, the controller disables the
corresponding current regulator. The current regulator
cannot be disabled by connecting the respective FB_
pin to V
CC
after the IC is powered up.
All FB_ pins in use are combined to extract a lowest
FB_ voltage (LVC) (see Figure 2). LVC is fed into the
step-up converter’s error amplifier and is used to set
the output voltage.
Current-Source Fault Protection
The MAX17061 performs a diagnostic test at startup.
Open/short strings are disabled. LED fault open/short is
also detected after startup. When one or more strings
fails after startup, the corresponding current sources
are disabled. The remaining LED strings still operate
normally. When a fault is detected, bit 4 or/and bit 5 of
the fault/status resister are set (see the
Fault/Status
Register
section).
Open-Current Source Protection
The MAX17061 step-up converter output voltage is reg-
ulated according to the minimum FB_ voltages on all
the strings in use. If one or more strings are open, the
respective FB_ pins are pulled to ground. For any FB_
lower than 175mV, the corresponding current source is
disabled. The unaffected LED strings still operate nor-
mally. If all strings in use are open, the MAX17061
shuts down the step-up converter.
The MAX17061 can tolerate A slight mismatch (4.4V)
between LED strings. When severe mismatches
(> 4.4V) or WLED shorts occur, the FB_ voltages will be
uneven because mismatched voltage drops across
strings. If a given FB_ voltage is higher than 5.6V (typ)
after 24µs blanking time when LEDs are turned on, an
LED short condition is detected on the respective
string. When the short continues for greater than 2ms,
the string is disabled. The controller allows the unaf-
fected LED strings to operate normally. The LED short-
protection feature is disabled during the soft-start
phase of the step-up converter.
Dimming Control
The MAX17061 internally generates a DPWM signal for
accurate WLED brightness dimming control. The DPWM
frequency is adjustable through an external setting
resistor and has 1.5% accuracy for R
FSET
= 464kΩ. The
duty cycle of this DPWM signal can be controlled exter-
nally through two interfaces: PWM and SMBus. The
ISET pin sets the amplitude of the current sources for
each LED string (Figure 4). The internal DPWM signal
directly controls the duty cycle of these current sources.
The resulting current is chopped and synchronized to
the DPWM signal. When filtered by the slow response
time of the human eye, the overall brightness is modu-
lated in a consistent flicker-free manner.
MAX17061
8-String White LED Driver with
SMBus for LCD Panel Applications
______________________________________________________________________________________ 15
Table 1. Frequency Selection
