Rainbow Electronics MAX1839 - Manual (Page 13)

MAX1739/MAX1839
†
Wide Brightness Range
CCFL Backlight Controllers
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allow the circuit to operate in dropout at extremely low
battery voltages where the backlight’s performance is
secondary. All backlight circuit designs can undergo a
transient overvoltage condition when the laptop is
plugged into the AC adapter and V
BATT suddenly
increases. The MAX1739/MAX1839 contain a unique
clamp circuit on V
CCI . Along with the feed-forward cir-
cuitry, it ensures that there is not a transient transformer
overvoltage when leaving dropout.
The PK_DET_CLAMP circuit limits V CCI to the peaks of
the buck-regulator’s PWM ramp generator. As the cir-
cuit reaches dropout, V
CCI approaches the peaks of
the PWM ramp generator in order to reach maximum
duty cycle. If V
BATT decreases further, the control loop
loses regulation and V CCI tries to reach its positive sup-
ply rail. The clamp circuit on V CCI keeps this from hap-
pening, and V CCI rides just above the peaks of the
PWM ramp. As V BATT decreases further, the feed-for-
ward PWM ramp generator loses amplitude and the
clamp drags V
CCI down with it to a voltage below
where V CCI would have been if the circuit was not in
dropout. When VBATT is suddenly increased out of
dropout, V
CCI is still low and maintains the drive on the
transformer at the old dropout level. The circuit then
slowly corrects and increases V
CCI to bring the circuit
back into regulation.
Buck Regulator
The buck regulator uses the signals from the PWM
comparator, the current-limit detection on CS, and
DPWM signals to control the high-side MOSFET duty
cycle. The regulator uses voltage-mode PWM control
and is synchronized to the Royer oscillator. A falling
edge on SYNC turns on the high-side MOSFET after a
375ns minimum off-time delay. The PWM comparator or
the CS current limit ends the on-cycle.
Interface Selection
Table 1 lists the functionality of SH/SUS, CRF/SDA, and
CTL/SCL in each of the three interface modes of the
MAX1739/MAX1839. The MAX1739 features both an
SMBus digital interface and an analog interface, while
the MAX1839 features only the analog interface. Note
that MODE can also synchronize the DPWM frequency
(see Synchronizing the DPWM Frequency).
Dimming Range
Brightness is controlled by either the analog interface
(see Analog Interface) or the SMBus interface (see
SMBus Interface). CCFL brightness is adjusted in three
ways:
1) Lamp current control, where the magnitude of the
average lamp current is adjusted.
2) DPWM control, where the average lamp current is
pulsed to the lamp with a variable duty cycle.
3) A combination of the first two methods.
In each of the three methods, a 5-bit brightness code is
generated from the selected interface and is used to
set the lamp current and/or DPWM duty cycle.
The 5-bit brightness code defines the lamp current
level with ob00000 representing minimum lamp current
and ob11111 representing maximum lamp current. The
average lamp current is measured across an external
sense resistor (see Sense Resistors). The voltage on
the sense resistor is measured at CSAV. The brightness
code adjusts the regulation voltage at CSAV (V
CSAV ).
The minimum average V CSAV is V MINDAC /10, and the
maximum average is set by the following formula:
V CSAV = V REF ✕ 31 / 320 + V MINDAC / 320
which is between 193.75mV and 200mV.
Note that if V CSAV does not exceed 100mV peak (which
is about 32mV average) for over 2 seconds, the
MAX1739/MAX1839 will assume a lamp-out condition
and shut down (see Lamp-Out Detection).
The equation relating brightness code to C SAV regula-
tion voltage is:
V CSAV = V REF
✕ n / 320 + V MINDAC
✕ (32 - n) / 320
where n is the brightness code.
To always use maximum average lamp current when
using DPWM control, set V
MINDAC to V REF .
DPWM control works similar to lamp current control in
that it also responds to the 5-bit brightness code. A
DIGITAL
INTERFACE
ANALOG INTERFACE
PIN
MODE = VL
(MAX1739 only) MODE = REF,
V
CTL /SCL = 0 = maximum brightness
MODE = GND,
V
CTL /SCL = 0 = minimum brightness
SH/SUS SMBus suspend Logic-level shutdown control input
CRF/SDA SMBus data I/O Reference input for minimum brightness Reference input for maximum brightness
CTL/SCL SMBus clock input Analog control input to set brightness (range from 0 to CRF/SDA)
Table 1. Interface Modes