MAX16833 Datasheet, PDF (13/22 Pages) Maxim Integrated Products – High-Voltage HB LED Drivers with Integrated High-Side Current Sense

High-Voltage HB LED Drivers with
Integrated High-Side Current Sense
SYNC
C1
680pF
R2
22I
C2
1000pF
D2
SD103AWS
RT PIN
RRT
24.9I
GND
GND
Figure 1. SYNC Circuit
Figure 1 shows the frequency-synchronization circuit
suitable for applications where a 5V amplitude pulse with
20% to 80% duty cycle is available as the synchroniza-
tion source. This circuit can be used for SYNC frequen-
cies in the 100kHz to 1MHz range. C1 and R2 act as a
differentiator that reduces the input pulse width to suit
the MAX16833 RT/SYNC input. D2 bypasses the nega-
tive current through C1 at the falling edge of the SYNC
source to limit the minimum voltage at the RT/SYNC pin.
The differentiator output is AC-coupled to the RT/SYNC
pin through C2.
The output impedance of the SYNC source should be
low enough to drive the current through R2 on the rising
edge. The rise/fall times of the SYNC source should be
less than 50ns to avoid excessive voltage drop across
C1 during the rise time. The amplitude of the SYNC
source can be between 4V and 5V. If the SYNC source
amplitude is 5V and the rise time is less than 20ns, then
the maximum peak voltage at RT/SYNC pin can get
close to 6V. Under such conditions, it is desirable to
use a resistor in series with C1 to reduce the maximum
voltage at the RT/SYNC pin. For proper synchronization,
the peak SYNC pulse voltage at RT/SYNC pin should
exceed 3.8V.
Frequency Dithering (LFRAMP/MAX16833)
The MAX16833 features a low-frequency ramp output.
Connect a capacitor from LFRAMP to ground to program
the ramp frequency. Connect to SGND if not used. A
resistor can be connected between LFRAMP and RT/
SYNC to dither the PWM switching frequency to achieve
spread spectrum. A lower value resistor provides a larg-
er amount of frequency dithering. The LFRAMP voltage
is a triangular waveform between 1V (typ) and 2V (typ).
The ramp frequency is given by:
f_LFRAMP(Hz)
= ^50FA
C _LFRAMP (F)
Voltage-Reference Output
(REF/MAX16833B)
The MAX16833B has a 2% accurate 1.64V reference
voltage on the REF output. Connect a 1FF ceramic
capacitor from REF to SGND to provide a stable refer-
ence voltage. This reference can supply up to 80µA. This
output can drive a resistive divider to the ICTRL input
for analog dimming. The resistance from REF to ground
should be greater than 20.5kI.
Switching MOSFET
Current-Sense Input (CS)
CS is part of the current-mode control loop. The switch-
ing control uses the voltage on CS, set by RCS (R4 in the
Typical Operating Circuits) and RSLOPE (R1 in the Typical
Operating Circuits), to terminate the on pulse width of the
switching cycle, thus achieving peak current-mode con-
trol. Internal leading-edge blanking of 50ns is provided
to prevent premature turn-off of the switching MOSFET
in each switching cycle. Resistor RCS is connected
between the source of the n-channel switching MOSFET
and PGND.
During switching, a current ramp with a slope of 50FA
x fSW is sourced from the CS input. This current ramp,
along with resistor RSLOPE, programs the amount of slope
compensation.
Overvoltage-Protection Input (OVP)
OVP sets the overvoltage-threshold limit across the
LEDs. Use a resistive divider between ISENSE+ to OVP
and SGND to set the overvoltage-threshold limit. An
internal overvoltage-protection comparator senses the
differential voltage across OVP and SGND. If the dif-
ferential voltage is greater than 1.23V, NDRV goes low,
DIMOUT goes high, and FLT asserts. When the differen-
tial voltage drops by 70mV, NDRV is enabled, DIMOUT
goes low, and FLT deasserts.
Fault Indicator (FLT)
The MAX16833/MAX16833B feature an active-low, open-
drain fault indicator (FLT). FLT goes low when one of the
following conditions occur:
U Overvoltage across the LED string
U Short-circuit condition across the LED string
U Overtemperature condition
FLT goes high when the fault condition ends.
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