MAX16807EVKIT+ Maxim Integrated Products, MAX16807EVKIT+ Datasheet - Page 7

• Manufacturer Part Number: MAX16807EVKIT+
• Description: EVAL KIT FOR MAX16807
• Manufacturer: Maxim Integrated Products

Specifications of MAX16807EVKIT+

• Current - Output / Channel: 50mA
• Outputs And Type: 8, Non-Isolated
• Voltage - Output: 32V
• Features: Dimmable
• Voltage - Input: 9 ~ 16V
• Utilized Ic / Part: MAX16807
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

by the feedback loop, the output pulse will terminate
sooner than normally expected. The minimum slope
compensation that should be added to stabilize the
current loop is half of the worst-case (maximum) falling
slope of inductor current.
Adding a ramp, with positive slope in sync with the
switching frequency, to the current-sense signal can
produce the desired function. The greater the duty
cycle, the greater the added voltage, and the greater
the difference between the set current and the actual
inductor current. In the MAX16807 EV kit, the oscillator
ramp signal is buffered using Q1 and added to the cur-
rent-sense signal with proper scaling to implement the
slope compensation. Follow the steps below to calcu-
late the component values for slope compensation.
Calculate the worst-case falling slope of the inductor
current using the following equation:
From the inductor current falling slope, find its equiva-
lent voltage slope across the current-sense resistor
R
The minimum voltage slope that should be added to
the current-sense waveform is half of V
ing stability up to 100% duty cycle. As the maximum
continuous duty cycle used is less than 100%, the mini-
mum required compensation slope becomes:
where the factor 1.1 provides a 10% margin. Resistors
R5 and R6 determine the attenuation of the buffered
voltage slope from the emitter of Q1. The forward drop
of signal diode D7, together with the V
cancel the 1.1V offset of the ramp waveform. Calculate
the approximate slope of the oscillator ramp using the
following equation:
where 1.7V is the ramp amplitude and F
switching frequency.
Select the value of R5 so that the input bias current of
the current-sense comparators does not add consider-
able error to the current-sense signal. The value of R6
for the slope compensation is given by the equation:
CS
(R8 parallel with R9) using the following equation:
VC
IL
SLOPE
SLOPE
V
SLOPE
VR
_______________________________________________________________________________________
=
=
SLOPE
(
VLED
V
SLOPE
=
IL
MAX
=
SLOPE
1 7 .
×
L
(
D
+
2
MIN
×
MAX
D
V
F
MAX
D
×
SW
R
−
CS
VIN
− ×
BE
SLOPE
1 1 1
)
MIN
of Q1, almost
)
.
SW
for ensur-
is the
MAX16807 Evaluation Kit
Like any other circuit with feedback, the boost convert-
er that generates the voltage for the LED strings needs
to be compensated for stable control of its output volt-
age. When the boost converter is operated in CCM,
there exists a right-half-plane (RHP) zero in the power-
circuit transfer function. This zero adds a 20dB/decade
gain, together with a 90° phase lag, which is difficult to
compensate. The easiest way to avoid this zero is to roll
off the loop gain to 0dB, at a frequency less than half of
the RHP zero frequency, with a -20dB/decade slope.
For a boost converter, the worst-case RHP zero fre-
quency (F
where D
tance of the inductor, and I
which is the sum of all the LED string currents.
The boost converter used in the MAX16807 EV kit is
operated in peak current-mode control. There are two
feedback loops within a current-mode controlled con-
verter: an inner loop that controls the inductor current,
and an outer loop that controls the output voltage. The
amplified voltage error produced by the outer voltage
loop is the input to the inner current loop that controls
the peak inductor current.
The internal current loop converts the double-pole/sec-
ond-order system, formed by the inductor and the out-
put capacitor C
single pole consisting of the output filter capacitor and
the output load. As the output load is a constant current
(very high Thevenin impedance), this pole is located
near the origin (0Hz). The phase lag created by the
output pole for any frequency will be 90°. However, as
the power circuit DC gain is limited by other factors, the
gain starts falling at -20dB/decade from a non-zero fre-
quency before which the power circuit gain will be stable.
Total gain of the feedback loop at DC is given by the
following equation:
where G
error-amplifier open-loop DC gain, typically 100dB. G
is the gain of the feedback network for adaptive control
of the VLED, which is seen from VLED to the error-
amplifier input (FB pin). The adaptive control senses
MAX
P
ZRHP
is the power-circuit DC gain and G
is the maximum duty cycle, L is the induc-
F
) is given by the following equation:
ZRHP
R
G
OUT
6
TOT
=
⎛
⎜
⎝
, to a first-order system having a
=
VC
VR
=
Feedback Compensation
VLED
G
SLOPE
SLOPE
P
×
2
π
G
(
1
O
× ×
EA
−
−
L I
is the output current,
D
1
×
⎞
⎟ ×
⎠
MAX
G
O
FB
R
5
)
2
EA
is the
FB
7