Datasheet

16 Maxim Integrated
High-Voltage HB LED Drivers with
Integrated High-Side Current Sense
MAX16833/MAX16833B–MAX16833D
Slope Compensation
Slope compensation should be added to converters
with peak current-mode control operating in continuous-
conduction mode with more than 50% duty cycle to
avoid current-loop instability and subharmonic oscilla-
tions. The minimum amount of slope compensation that
is required for stability is:
V
SCMIN
= 0.5 (inductor current downslope -
inductor current upslope) x R4
In the ICs, the slope-compensating ramp is added to the
current-sense signal before it is fed to the PWM com-
parator. Connect a resistor (R1) from CS to the inductor
current-sense resistor terminal to program the amount of
slope compensation.
The ICs generate a current ramp with a slope of 50FA/
t
OSC
for slope compensation. The current-ramp signal is
forced into the external resistor (R1) connected between
CS and the source of the external MOSFET, thereby
adding a programmable slope compensating voltage
(V
SCOMP
) at the current-sense input CS. Therefore:
dV
SC
/dt = (R1 x 50FA)/t
OSC
in V/s
The minimum value of the slope-compensation voltage
that needs to be added to the current-sense signal at
peak current and at minimum line voltage is:
MAX LED INMIN
MIN
MIN SW
(D (V - 2V ) R4)
SC (V)Boost
2L f
××
=
××
MAX LED INMIN
MIN
MIN SW
(D (V - V ) R4)
SC (V)Buck-boost
2L f
××
=
××
where f
SW
is the switching frequency, D
MAX
is the maxi-
mum duty cycle, which occurs at low line, V
INMIN
is the
minimum input voltage, and L
MIN
is the minimum value of
the selected inductor. For adequate margin, the slope-com-
pensation voltage is multiplied by a factor of 1.5. Therefore,
the actual slope-compensation voltage is given by:
V
SC
= 1.5SC
MIN
From the previous formulas, it is possible to calculate the
value of R4 as:
For boost configuration:
LED INMIN
P MAX
MIN SW
0.418V
R4
V 2V
IL 0.75D
Lf
−
=
+
For buck-boost configuration:
LED INMIN
P MAX
MIN SW
0.418V
R4
VV
IL 0.75D
Lf
−
=
+
The minimum value of the slope-compensation resistor
(R1) that should be used to ensure stable operation at
minimum input supply voltage can be calculated as:
For boost configuration:
LED INMIN
MIN SW
(V 2V ) R4 1.5
R1
2 L f 50 A
− ××
=
× × ×µ
For buck-boost configuration :
LED INMIN
MIN SW
(V V ) R4 1.5
R1
2 L f 50 A
− ××
=
× × ×µ
where f
SW
is the switching frequency in hertz, V
INMIN
is the minimum input voltage in volts, V
LED
is the LED
voltage in volts, D
MAX
is the maximum duty cycle, IL
P
is the peak inductor current in amperes, and L
MIN
is the
minimum value of the selected inductor in henries.
Output Capacitor
The function of the output capacitor is to reduce the
output ripple to acceptable levels. The ESR, ESL, and
the bulk capacitance of the output capacitor contribute
to the output ripple. In most applications, the output ESR
and ESL effects can be dramatically reduced by using
low-ESR ceramic capacitors. To reduce the ESL and
ESR effects, connect multiple ceramic capacitors in par-
allel to achieve the required bulk capacitance. To mini-
mize audible noise generated by the ceramic capacitors
during PWM dimming, it could be necessary to minimize
the number of ceramic capacitors on the output. In these
cases, an additional electrolytic or tantalum capacitor
provides most of the bulk capacitance.
Boost and Buck-Boost Configurations
The calculation of the output capacitance is the same for
both boost and buck-boost configurations. The output rip-
ple is caused by the ESR and the bulk capacitance of the
output capacitor if the ESL effect is considered negligible.
For simplicity, assume that the contributions from ESR and
the bulk capacitance are equal, allowing 50% of the ripple
for the bulk capacitance. The capacitance is given by:
LED MAX
OUT
OUTRIPPLE SW
I 2D
C
Vf
××
≥
×