Datasheet

12 Maxim Integrated
High-Voltage HB LED Drivers with
Integrated High-Side Current Sense
MAX16833/MAX16833B–MAX16833D
Analog Dimming (ICTRL)
The ICs offer an analog dimming control input (ICTRL).
The voltage at ICTRL sets the LED current level when
V
ICTRL
< 1.2V. The LED current can be linearly adjusted
from zero with the voltage on ICTRL. For V
ICTRL
> 1.4V,
an internal reference sets the LED current. The maximum
withstand voltage of this input is 5.5V.
Low-Side Linear Regulator (V
CC
)
The ICs feature a 7V low-side linear regulator (V
CC
).
V
CC
powers up the switching MOSFET driver with sourc-
ing capability of up to 50mA. Use a 1FF (min) low-ESR
ceramic capacitor from V
CC
to PGND for stable opera-
tion. The V
CC
regulator goes below 7V if the input volt-
age falls below 7V. The dropout voltage for this regulator
at 50mA is 0.2V. This means that for an input voltage of
5V, the V
CC
voltage is 4.8V. The short-circuit current on
the V
CC
regulator is 100mA (typ). Connect V
CC
to IN if
V
IN
is always less than 7V.
LED Current-Sense Inputs (ISENSE±)
The differential voltage from ISENSE+ to ISENSE- is fed
to an internal current-sense amplifier. This amplified sig-
nal is then connected to the negative input of the trans-
conductance error amplifier. The voltage-gain factor of
this amplifier is 6.15.
The offset voltage for this amplifier is P 1mV.
Internal Transconductance Error Amplifier
The ICs have a built-in transconductance amplifier used
to amplify the error signal inside the feedback loop.
When the dimming signal is low, COMP is disconnected
from the output of the error amplifier and DIMOUT goes
high. When the dimming signal is high, the output of
the error amplifier is connected to COMP and DIMOUT
goes low. This enables the compensation capacitor to
hold the charge when the dimming signal has turned off
the internal switching MOSFET gate drive. To maintain
the charge on the compensation capacitor C
COMP
(C4
in the Typical Operating Circuits), the capacitor should
be a low-leakage ceramic type. When the internal dim-
ming signal is enabled, the voltage on the compensation
capacitor forces the converter into steady state almost
instantaneously.
Internal Oscillator (RT/SYNC)
The internal oscillators of the ICs are programmable from
100kHz to 1MHz using a single resistor at RT/SYNC.
Use the following formula to calculate the switching fre-
quency:
( )
( )
OSC
RT
OSC
RT
7350 k
f (kHz) for the MAX16833 MAX16833B
R (k )
6929 k
f (kHz) for the MAX16833C MAX16833D
R (k )
Ω
=
Ω
Ω
=
Ω
where R
RT
is the resistor from RT/SYNC to SGND.
Synchronize the oscillator with an external clock by
AC-coupling the external clock to the RT/SYNC input.
For f
OSC
between 200kHz and 1MHz, the capacitor used
for the AC-coupling should satisfy the following relation:
-6
-9
SYNC
RT
9.8624 10
C 0.144 10 farads
R
×
≤ −×
where R
RT
is in kω. For f
OSC
below 200GHz, C
SYNC
≤
268nF.
The pulse width for the synchronization pulse should
satisfy the following relations:
CLK
PW PW
CLK S CLK OSC
1.05 t
tt
0.5
and 1-
tV t t

×
<<


PW
SS
CLK
t
3.4V 0.8 - V V 5V
t

< +<


where t
PW
is the synchronization source pulse width,
t
CLK
is the synchronization clock time period, t
OSC
is
the free-running oscillator time period, and V
S
is the syn-
chronization pulse-voltage level.
Ensure that the external clock signal frequency is at least
1.1 x f
OSC,
where f
OSC
is the oscillator frequency set
by R
RT
. A typical pulse width of 200ns can be used for
proper synchronization of a frequency up to 250kHz. A
rising external clock edge (sync) is interpreted as a syn-
chronization input. If the sync signal is lost, the internal
oscillator takes control of the switching rate returning the
switching frequency to that set by R
RT
. This maintains
output regulation even with intermittent sync signals.