MAX1739EEP+ Maxim Integrated Products, MAX1739EEP+ Datasheet - Page 13

• Manufacturer Part Number: MAX1739EEP+
• Description: Display Drivers Wide Brightness Rang e CCFL Backlight Con
• Manufacturer: Maxim Integrated Products

Specifications of MAX1739EEP+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
increases. The MAX1739/MAX1839 contain a unique
clamp circuit on V
cuitry, it ensures that there is not a transient transformer
overvoltage when leaving dropout.
The PK_DET_CLAMP circuit limits V
the buck-regulator’s PWM ramp generator. As the cir-
cuit reaches dropout, V
the PWM ramp generator in order to reach maximum
duty cycle. If V
loses regulation and V
ply rail. The clamp circuit on V
pening, and V
PWM ramp. As V
ward PWM ramp generator loses amplitude and the
clamp drags V
where V
dropout. When VBATT is suddenly increased out of
dropout, V
transformer at the old dropout level. The circuit then
slowly corrects and increases V
back into regulation.
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.
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
Table 1. Interface Modes
SH/SUS
CRF/SDA
CTL/SCL
PIN
CCI
CCI
would have been if the circuit was not in
is still low and maintains the drive on the
MODE = VL
(MAX1739 only)
SMBus suspend
SMBus data I/O
SMBus clock input
BATT
CCI
CCI
BATT
______________________________________________________________________________________
CCI
INTERFACE
rides just above the peaks of the
DIGITAL
decreases further, the control loop
down with it to a voltage below
. Along with the feed-forward cir-
CCI
decreases further, the feed-for-
CCI
tries to reach its positive sup-
approaches the peaks of
CCI
Interface Selection
CCI
keeps this from hap-
Buck Regulator
CCI
to bring the circuit
MODE = REF,
V
Logic-level shutdown control input
Reference input for minimum brightness
Analog control input to set brightness (range from 0 to CRF/SDA)
CTL
BATT
to the peaks of
/SCL = 0 = maximum brightness
suddenly
CCFL Backlight Controllers
Wide Brightness Range
that MODE can also synchronize the DPWM frequency
(see Synchronizing the DPWM Frequency).
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
The minimum average V
maximum average is set by the following formula:
which is between 193.75mV and 200mV.
Note that if V
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
tion voltage is:
where n is the brightness code.
To always use maximum average lamp current when
using DPWM control, set V
DPWM control works similar to lamp current control in
that it also responds to the 5-bit brightness code. A
V
CSAV
ANALOG INTERFACE
V
CSAV
= V
CSAV
REF
= V
REF
✕
MODE = GND,
V
Reference input for maximum brightness
does not exceed 100mV peak (which
CTL
n / 320 + V
✕
/SCL = 0 = minimum brightness
31 / 320 + V
CSAV
MINDAC
MINDAC
is V
to V
MINDAC
Dimming Range
MINDAC
✕
REF
(32 - n) / 320
.
/10, and the
/ 320
SAV
regula-
CSAV
13
).