Rainbow Electronics MAX16834 - Manual (Page 10)

MAX16834
High-Power LED Driver with Integrated High-Side LED
Current Sense and PWM Dimming MOSFET Driver
10 ______________________________________________________________________________________
Internal Oscillator
The internal oscillator of the MAX16834 is programma-
ble from 100kHz to 1MHz using a single resistor at
RT/SYNC. Use the following formula to calculate the
switching frequency:
where RT is the resistor from RT/SYNC to SGND.
The MAX16834 synchronizes to an external clock signal
at RT/SYNC. The application of an external clock dis-
ables the internal oscillator allowing the MAX16834 to
use the external clock for switching operation. The
internal oscillator is enabled if the external clock is
absent for more than 50µs. The synchronizing pulse
minimum width for proper synchronization is 200ns.
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 R
CS , to termi-
nate the on pulse width of the switching cycle, thus
achieving peak current-mode control. Internal leading-
edge blanking is provided to prevent premature turn-off
of the switching MOSFET in each switching cycle.
Slope Compensation (SC)
The MAX16834 uses an internal-ramp generator for
slope compensation. The ramp signal also resets at the
beginning of each cycle and slews at the rate pro-
grammed by the external capacitor connected at SC.
The current source charging the capacitor is 100µA.
Overvoltage Protection (OVP+)
OVP+ sets the overvoltage threshold limit across the
LEDs. Use a resistive divider between output OVP+
and LV to set the overvoltage threshold limit. An internal
overvoltage protection comparator senses the differen-
tial voltage across OVP+ and LV. If the differential volt-
age is greater than 1.435V, NDRV is disabled and FLT
asserts. When the differential voltage drops by 200mV,
NDRV is enabled and FLT deasserts. The PWM dim-
ming MOSFET is still controlled by the PWMDIM input.
Fault Indicator (
FLT)
The MAX16834 features an active-low, open-drain fault
indicator (FLT). FLT asserts when one of the following
occurs:
1) Overvoltage across the LED string
2) Short-circuit condition across the LED string, or
3) Overtemperature condition
When the output voltage drops below the overvoltage
set point minus the hysteresis, FLT deasserts. Similarly
during the short-circuit period, the fault signal
deasserts when the dimming MOSFET is on, which
happens every hiccup cycle during short circuit. During
overtemperature fault, the FLT signal is the inverse of
the PWM input.
Applications Information
Setting the UVLO Threshold
The UVLO threshold is set by resistors R1 and R2 (see
Figure 2). The MAX16834 turns on when the voltage
across R2 exceeds 1.435V, the UVLO threshold. Use
the following equation to set the desired UVLO thresh-
old:
In a typical application, use a 10k
Ω resistor for R2 and
then calculate R1 based on the desired UVLO threshold.
Setting the Overvoltage Threshold
The overvoltage threshold is set by resistors R4 and R9
(see Figure 2). The overvoltage circuit in the MAX16834
is activated when the voltage on OVP+ with respect to
LV exceeds 1.435V. Use the following equation to set
the desired overvoltage threshold:
Programming the LED Current
The LED current is programmed using the voltage on
REFI and the LED current-sense resistor R10 (see
Figure 2). The current is given by:
where V REF is 3.7V and the resistors R5, R6, and R10
are in ohms. The regulation voltage on the LED current-
sense resistor must not exceed 0.3V to prevent activa-
tion of the LED short-circuit protection circuit.
I
V R
R R R
A
LED
REF =
Ч
Ч + Ч
( ) 5
10 6 5 9 9 ( ) .
V V R R R
OV
= + 1 435 4 9 9 . ( ) /
V V R R R
UVEN
= +
1 435 1 2 2 . ( ) /
f (kHz)
5000k
RT(k )
(kHz)
OSC
= ×
Ω
Ω