Rainbow Electronics MAX8709 User Manual

Page 15

MAX8709

High-Efficiency CCFL Backlight

Controller with SMBus Interface

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POR and UVLO

The MAX8709 includes power-on-reset (POR) and
undervoltage-lockout (UVLO) circuits. The POR resets
all internal registers such as DAC outputs, fault latches,
and all SMBus registers. POR occurs when V

below 1.5V. The SMBus input logic thresholds are only
guaranteed to meet electrical characteristic limits for
V

as low as 3.5V, but the interface continues to func-

tion down to the POR threshold.

The UVLO is activated and disables both high-side and
low-side switch drivers when V

is below 4.2V (typ).

Low-Power Shutdown (SUS)

When the MAX8709 is placed in shutdown, all functions
of the IC are turned off except for the 5.3V linear regu-
lator that powers all internal registers and the SMBus
interface. The SMBus interface is accessible in shut-
down. In shutdown, the linear-regulator output voltage
drops to about 4.5V and the supply current is 6µA (typ),
which is the required power to maintain all internal reg-
ister states. While in shutdown, lamp-out detection and
short-circuit detection latches are reset. The device can
be placed into shutdown either by writing to the shut-
down-mode register or pulling SUS low.

Lamp-Out Protection

For safety, the MAX8709 monitors the lamp-current
feedback (IFB) to detect faulty or open CCFL tubes and
secondary short circuits in the lamp and IFB sense
resistor. If the voltage on IFB is continuously below 30%
of the LOT voltage for greater than 1.22s (typ), the
MAX8709 latches off the full bridge. Unlike the normal
shutdown mode, the linear-regulator output (V

)

remains at 5.3V. Toggling SUS or cycling the input
power reactivates the device.

During the 1.22s delay, V

CCI

slowly rises, increasing

in an attempt to maintain lamp current regulation.

As V

CCI

rises, V

CCV

rises with it until the secondary

voltage reaches its preset limit. At this point, V

CCV

stops and limits the secondary voltage by limiting t

Because V

CCV

is limited to 150mV above V

CCI

, the volt-

age control loop is able to quickly limit the secondary
voltage. Without this clamping feature, the transformer
voltage overshoots to dangerous levels because V

CCV

takes time to slew down from its supply rail.

Primary Overcurrent Protection (ILIM)

The MAX8709 senses primary current in each switching
cycle. When the regulator turns on the low-side MOSFET,
a comparator monitors the voltage drop from LX_ to GND.
If the voltage exceeds the current-limit threshold, the reg-
ulator turns off the high-side switch at the opposite side of
the primary to prevent the transformer primary current
from increasing further.

The current-limit threshold can be adjusted using the
ILIM input. Connect a resistive voltage-divider between
REF or V

and GND with the midpoint connected to

ILIM. The current-limit threshold measured between
LX_ and GND is 1/5th the voltage at ILIM. The ILIM
adjustment range is 0 to 3V. Connect ILIM to V

select the default current-limit threshold of 0.2V.

Secondary Current Limit (ISEC)

The secondary current limit provides failsafe current
limiting in case a failure, such as a short circuit or leak-
age from the lamp high-voltage terminal to ground, pre-
vents the CCI current control loop from functioning
properly. ISEC monitors the voltage across a sense
resistor placed between the transformer’s low-voltage
secondary terminal and ground. The ISEC voltage is
internally half-wave rectified and continuously com-
pared to the ISEC regulation threshold (1.25V typ). Any
time the ISEC voltage exceeds the threshold, a con-
trolled current is drawn from CCI to reduce the on-time
of the bridge’s high-side switches.

Reference Output (REF)

The reference output is nominally 2V, and can source at
least 40µA (see the Typical Operating Characteristics).
Bypass REF with a 0.22µF ceramic capacitor connected
between REF and GND.

Linear-Regulator Output (V

)

The internal linear regulator steps down the DC input
voltage to 5.3V (typ). The linear regulator supplies
power to the internal control circuitry of the MAX8709
and can also be used to power the MOSFET drivers by
connecting V

directly to V

. The V

voltage drops

to 4.5V in shutdown.

100

BRIGHTNESS CODE

DPWM DUTY CYCLE (%)

DPWM SETTINGS

Figure 6. DPWM Settings