Startup
At startup, the MAX8790A checks each FB_ pin to deter-
mine if the respective current string is enabled. Each FB_
pin is internally pulled up with a 180μA current source. If
an FB_ pin is connected to GND, the corresponding string
current source is disabled. This feedback scan takes
approximately 4.2ms, after which the step-up converter
begins switching.
Shutdown
When the SHDN pin is less than 0.8V, the MAX8790A
shuts down the internal LDO, the reference, current
sources, and all control circuitry. The resulting supply cur-
rent is less than 10μA. While the n-channel MOSFET is
turned off, the step-up regulator’s output is connected to
IN through the external inductor and rectifier diode.
Frequency Selection
A tri-level OSC input sets the internal oscillator frequency
for the step-up converter, as shown in Table 1. High-
frequency (1MHz) operation optimizes the regulator for
the smallest component size, at the expense of effi-
ciency 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 reason, the
MAX8790A features a dedicated overvoltage feedback
input (OV). The OV pin is connected to the center tap of a
resistive voltage-divider from the highvoltage output (see
Figure 1). When the MAX8790A is powered up, if none of
the LED strings on FB1–FB6 are connected to the step-
up converter output, the step-up converter regulates the
output voltage to VOUT = 1.23V(1 + R1 / R2). When VOV
exceeds 1.23V, a comparator turns off N1. The step-up
converter switch is reenabled after the output voltage
drops below the protection threshold.
LED Current Sources
Maintaining uniform LED brightness and dimming capa-
bility are critical for LCD backlight applications. The
MAX8790A is equipped with a bank of six matched
current sources. These specialized current sources are
accurate to within ±1.5% and can be switched on and off
within 10μs, enabling PWM frequencies of up to 2kHz. All
LED full-scale currents are identical and are set through
the ISET pin (15mA < ILED < 27mA).
The minimum voltage drop across each current source
is approximately 450mV at 20mA. 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 grounding
the respective FB_ pin at startup. When the IC is pow-
ered up, the controller scans settings for all FB_ pins. If
an FB_ pin is not grounded, an internal circuit pulls this
pin high, and the controller enables the corresponding
current source to regulate the string current. If the FB_
pin is grounded, the controller disables the correspond-
ing current regulator. The current regulator cannot be
disabled by grounding any of the FB_ pins after the IC is
powered up.
All FB_ pins in use are measured and the highest signal
(HVC) and the lowest signal (LVC) are extracted for two
feedback loops. HVC is used to identify excessive dis-
sipation across the current-source inputs. When HVC
is greater than VCC + 0.6V (typ) for greater than 65ms
(see the Current-Source Fault Protection section), a fault
latch is set and the MAX8790A is shut down. The LDO
output is not affected by the fault latch. LVC is fed into the
step-up converter’s error amplifier to regulate the step-up
converter’s output voltage.
Current-Source Fault Protection
The LED current sources are protected against string
open, short, and gross mismatch faults, using overvolt-
age detection circuitry on each FB_ pin. If any of these
three fault conditions persists for a preset duration, the
MAX8790A is latched off. The duration of the fault time
depends on the dimming mode and the duty cycle of
the BRT input (DBRT). In the DPWM mode, the timeout
interval is:
tTIMEOUT_DPWM = 65ms/DBRT
In analog dimming mode, the fault time is fixed at 65ms
for DBRT greater than 12.5%. When DBRT is less than
12.5%, the timeout interval is:
tTIMEOUT_ANALOG = 8.125ms/DBRT
The fault latch can be cleared by cycling the power or tog-
gling the shutdown pin SHDN.
Table 1. Frequency Selection
OSC SWITCHING FREQUENCY (kHz)
GND 500
Open 750
VCC 1000
MAX8790A Six-String White LED Driver with Active
Current Balancing for LCD Panel Applications
www.maximintegrated.com Maxim Integrated
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