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MAX16814
Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
7.35 × 10 Hz
R T =
7.72 × 10 9
R T =
The converter stops switching when the LED strings are
turned off during PWM dimming. The error amplifier is
disconnected from the COMP output to retain the com-
pensation capacitor charge . This allows the converter
to settle to steady-state level almost immediately when
the LED strings are turned on again. This unique feature
provides fast dimming response, without having to use
large output capacitors.
For the MAX16814A_ _ and the MAX16814U_ _, if the
PWM dimming on-pulse is less than or equal to five
switching cycles, the feedback controls the voltage on
OVP so that the converter output voltage is regulated at
95% of the OVP threshold. This mode ensures that narrow
PWM dimming pulses are not affected by the response
time of the converter. During this mode, the error amplifier
remains connected to the COMP output continuously and
the DC-DC converter continues switching.
Undervoltage Lockout (UVLO)
The MAX16814 features two undervoltage lockouts that
monitor the input voltage at IN and the output of the inter-
nal LDO regulator at V CC . The device turns on after both
V IN and V CC exceed their respective UVLO thresholds.
The UVLO threshold at IN is 4.3V when V IN is rising and
4.15V when V IN is falling. The UVLO threshold at V CC
is 4V when V CC is rising and 3.9V when V CC is falling.
Enable
EN is a logic input that completely shuts down the
device when connected to logic-low, reducing the cur-
rent consumption of the device to less than 40 F A. The
logic threshold at EN is 1.23V (typ). The voltage at EN
must exceed 1.23V before any operation can com-
mence. There is a 50mV hysteresis on EN. The EN input
also allows programming the supply input UVLO thresh-
old using an external voltage-divider to sense the input
voltage as shown below.
soft-start, the DC-DC converter output ramps towards
95% of the OVP voltage and uses feedback from the OVP
input. Soft-start terminates when the minimum current-sink
voltage reaches 1V or when the converter output reaches
95% OVP. The typical soft-start period is 100ms. The 1V
minimum OUT_ voltage is detected only when the LED
strings are enabled by PWM dimming. Connect OVP to
the boost converter output through a resistive divider
network (see the Typical Operating Circuit ).
When there is an open-LED condition, the converter output
hits the OVP threshold. After the OVP is triggered, open-
LED strings are disconnected and, at the beginning of the
dimming PWM pulse, control is transferred to the adaptive
voltage control. The converter output discharges to a level
where the new minimum OUT_ voltage is 1V.
Oscillator Frequency/External Synchronization
The internal oscillator frequency is programmable
between 200kHz and 2MHz using a resistor (R T ) con-
nected from the RT input to SGND. Use the equation
below to calculate the value of R T for the desired switch-
ing frequency, f SW .
9
f SW
(for the MAX16814A_ _ and the MAX16814U_ _).
f SW
(for the MAX16814B_ _ _).
Synchronize the oscillator with an external clock by
AC-coupling the external clock to the RT input. The
capacitor used for the AC-coupling should satisfy the
following relation:
C SYNC ≤ ?
-0.144 × 10 -3 ? ( μ F )
= ? ? × R2
? V UVLO ?
Use the following equation to calculate the value of R1
and R2 in Figure 3:
R1 - 1
? 1.23V ?
? 9.862
? R T
V IN
?
?
where V UVLO is the desired undervoltage lockout level
and 1.23V is the EN input reference. Connect EN to IN
if not used.
Soft-Start
The MAX16814 provides soft-start with internally set timing. At
MAX16814
1.23V
EN
R1
R2
power-up, the MAX16814 enters soft-start once unused LED
strings are detected and disconnected (see the Open-LED
Management and Overvoltage Protection section). During
Maxim Integrated
Figure 3. Setting the MAX16814 Undervoltage Lockout
Threshold
13
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