MAX16809EVKIT+ Maxim Integrated Products, MAX16809EVKIT+ Datasheet - Page 4

• Manufacturer Part Number: MAX16809EVKIT+
• Description: Power Management Modules & Development Tools EVAL KIT FOR MAX16809
• Manufacturer: Maxim Integrated Products

Specifications of MAX16809EVKIT+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

As the boost converter is operated in CCM, only part of
the stored energy in the inductor is discharged to VLED.
The advantages of CCM include reduced input and out-
put filtering, reduced EMI due to lower peak currents,
and higher converter efficiency. However, these advan-
tages come at the cost of a right-half-plane zero in the
converter-transfer function. Compensating this zero
requires reducing the system bandwidth, which affects
the converter-dynamic response. As the 16-channel,
constant-current-sink outputs control the current through
the LEDs, slower control of VLED does not affect the
LED operation. Compensation of the feedback circuit is
explained in the Feedback Compensation section.
An internal comparator turns off the gate pulse to the
external MOSFET if the voltage at the CS pin exceeds
0.3V. The current through the inductor that produces
0.3V at the CS pin is the maximum inductor current
possible (the actual current can be a little higher than
this limit due to the 60ns propagation delay from the CS
pin to the MOSFET drive output). This condition can
happen when the feedback loop is broken, when the
output capacitor charges during power-up, or when
there is an overload at the output. This feature protects
the MOSFET by limiting the maximum current passing
through it during such conditions.
The RC filter, consisting of R9 and C10, removes the
voltage spike across the current-sense resistors pro-
duced by the turn-on gate current of the MOSFET and
the reverse-recovery current of D2. Without filtering,
these current spikes can cause sense comparators to
falsely trigger and turn off the gate pulse prematurely.
The filter time constant should not be higher than
required (the MAX16809 EV kit uses a 120ns time con-
stant), as a higher time constant adds additional delay
to the current-sense voltage, effectively increasing the
current limit.
During normal operating conditions, the feedback loop
controls the peak current. The error amplifier compares
a scaled-down version of the LED supply voltage
(VLED) with a highly accurate 2.5V reference. The error
amplifier and compensation network then amplify the
error signal, and the current comparator compares this
signal to the sensed-current voltage to create a PWM
drive output.
Initially, decide the input supply voltage range, output
voltage VLED (the sum of the maximum LED total for-
ward voltage and 1V bias voltage for the constant-cur-
rent-sink output), and the output current I
of all the LED string currents).
MAX16809 Evaluation Kit
4
_______________________________________________________________________________________
Power-Circuit Design
OUT
(the sum
Calculate maximum duty cycle D
equation:
where V
(~0.6V), VIN
this case, 9V), and V
voltage of the MOSFET Q2 when it is on.
Select the switching frequency F
space, noise, dynamic response, and efficiency con-
straints. Select the maximum peak-to-peak ripple on
the inductor current IL
F
tor current. Use the following equations to calculate the
maximum average-inductor current IL
inductor current IL
Since IL
IL
Calculate the minimum inductance value L
inductor current ripple set to the maximum value:
Choose an inductor that has a minimum inductance
greater than this calculated value.
Calculate the current-sense resistor (R12 in parallel
with R13) using the equation below:
where 0.3V is the maximum current-sense signal volt-
age. The factor 0.75 is for compensating the reduction
of maximum current-sense voltage due to the addition
of slope compensation. Check this factor and adjust
after the slope compensation is calculated. See the
Slope Compensation section for more information.
SW
AVG
is 350kHz and IL
:
D
PP
is the forward drop of the rectifier diode D2
L
is ±30% of the average-inductor current
MIN
D
MIN
MAX
IL
IL
is the minimum input supply voltage (in
=
PEAK
PP
IL
PEAK
=
(
R
VIN
AVG
VLED V
=
FET
CS
PP
VLED V
IL
:
=
MIN
PP
AVG
=
=
is ±30% of the average induc-
IL
is the average drain-to-source
F
0 3 0 75
. For the MAX16809 EV kit,
1
SW
+
AVG
IL
.
−
−
+
I
×
OUT
PEAK
D
V
D
×
FET
×
0 3 2
D
MAX
−
+
.
IL
MAX
.
−
VIN
IL
PP
×
)
V
2
PP
×
FET
MIN
SW
D
using the following
MAX
AVG
based on the
MIN
and peak
with the