MAX8759 Maxim Integrated Products, MAX8759 Datasheet - Page 19

• Manufacturer Part Number: MAX8759
• Description: CCFL Backlight Controller
• Manufacturer: Maxim Integrated Products

For safety, the MAX8759 monitors the lamp-current
feedback inputs (IFB1 and IFB2) to detect faulty or
open CCFL tubes. As described in the Lamp-Current
Regulation section, the voltage on IFB1 and IFB2 is
internally full-wave rectified. If the rectified IFB1 or IFB2
voltage is below 600mV, the MAX8759 charges the
TFLT capacitor with 1µA. The MAX8759 sets the fault
latch and the device is shut down when the voltage on
TFLT exceeds 4V. Unlike the normal shutdown mode,
the linear regulator output (V
Clearing bit 0 of the device control register (0x01) or
cycling the input power clears the fault latch.
During the fault-delay period, the current control loop
tries to maintain the lamp-current regulation by increas-
ing the high-side MOSFET on-time. Because the lamp
impedance is very high when it is open, the transformer
secondary voltage rises as a result of the high Q-factor
of the resonant tank. Once the secondary voltage
exceeds the overvoltage threshold, the MAX8759 turns
on a 1000µA current source that discharges the COMP
capacitor. The on-time of the high-side MOSFET is
reduced, lowering the secondary voltage as the COMP
voltage decreases. Therefore, the peak voltage of the
transformer secondary winding never exceeds the limit
during the lamp-out delay period.
The MAX8759 senses primary current in each switch-
ing cycle. When the regulator turns on the low-side
MOSFET, a comparator monitors the voltage drop from
LX_ to PGND_. If the voltage exceeds the current-limit
threshold (430mV, typ), the regulator immediately turns
off the high-side switch to prevent the transformer pri-
mary current from increasing further.
Figure 7. Normalized Brightness vs. ALS Voltage
Low-Cost, SMBus, CCFL Backlight Controller
1.0
0.8
0.6
0.4
0.2
0
0
______________________________________________________________________________________
0.4
Primary Overcurrent Protection
0.8
V
ALS
(V)
1.2
CC
Fault Protections
Lamp-Out Protection
) remains at 5.35V.
1.6
2.0
The MAX8759 reduces the voltage stress on the trans-
former’s secondary winding by limiting the secondary
voltage during startup and open-lamp faults. The AC
voltage across the transformer secondary winding is
sensed through a capacitive voltage-divider formed by
C4 and C5 in Figure 1. The voltage across C5 is fed to
the VFB input. An overvoltage comparator compares
the VFB peak voltage with a 2.3V (typ) internal thresh-
old. Once the VFB peak voltage exceeds the overvolt-
age threshold, the MAX8759 turns on an internal
1000µA current source that discharges the COMP
capacitor. The high-side MOSFET’s on-time shortens as
the COMP voltage decreases, limiting the transformer
secondary’s peak voltage at the threshold determined
by the capacitive voltage-divider.
The MAX8759 senses the VFB voltage for undervoltage
condition. During the DPWM ON period, if the VFB volt-
age is below the undervoltage threshold (230mV, typ)
continuously for an internal delay period (250µs typ, for
R
The secondary current limit provides fail-safe current
limiting in case of a short circuit or leakage from the
lamp high-voltage terminal to ground that prevents the
current control loop from functioning properly. ISEC
monitors the voltage across a sense network placed
between the transformer’s low-voltage secondary termi-
nal and ground. The ISEC voltage is continuously com-
pared to the ISEC regulation threshold (1.21V, typ). Any
time the ISEC voltage exceeds the threshold, the
MAX8759 turns on a 1000µA current source that dis-
charges the COMP capacitor, reducing the on-time of
the high-side switches. At the same time, the MAX8759
charges the TFLT capacitor with a 135µA current. The
MAX8759 sets the fault latch and shuts down when the
voltage on TFLT exceeds 4V. Clearing bit 0 of the
device control register (0x01) or cycling the input
power clears the fault latch.
The internal linear regulator steps down the DC input
voltage at BATT pin to 5.35V (typ). The linear regulator
supplies power to the internal control circuitry of the
MAX8759 and is also used to power the MOSFET dri-
vers by connecting V
to 4.5V in shutdown.
The MAX8759 includes power-on reset (POR) and
undervoltage lockout (UVLO) features. POR resets the
fault latch and sets all the SMBus registers to their POR
FREQ
= 169kΩ), the MAX8759 shuts down.
Secondary Undervoltage Protection (VFB)
Linear Regulator Output (V
Secondary Voltage Limiting (VFB)
CC
Secondary Current Limit (ISEC)
to V
DD
. The V
POR and UVLO
CC
voltage drops
CC
19
)