MAX16809EVKIT+ Maxim Integrated Products, MAX16809EVKIT+ Datasheet - Page 6

• 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

MAX16809 Evaluation Kit
When the boost converter operates in CCM with more
than 50% duty cycle, subharmonic oscillations occur if
slope compensation is not implemented. Subharmonic
oscillations do not allow the PWM duty cycle to settle to
a peak current value set by the voltage-feedback loop.
The duty cycle oscillates back and forth about the
required value, usually at half the switching frequency.
Subharmonic oscillations die out if a sufficient negative
slope is added to the inductor peak current. This
means that for any peak current set by the feedback
loop, the output pulse terminates sooner than normally
expected. The minimum slope compensation that
should be added to stabilize the current loop is half of
the worst-case (max) falling slope of inductor current.
Adding a ramp to the current-sense signal, with posi-
tive slope in sync with the switching frequency, 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 MAX16809 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
(R12 parallel with R13) using the following equation:
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:
The factor 1.1 provides a 10% margin. Resistors R9
and R10 determine the attenuation of the buffered volt-
age slope from the emitter of Q1. The forward drop of
6
_______________________________________________________________________________________
VC
IL
SLOPE
SLOPE
V
SLOPE
=
=
(
VLED
V
SLOPE
=
IL
MAX
SLOPE
Slope Compensation
×
L
(
D
2
+
MIN
MAX
D
V
MAX
×
D
R
−
CS
VIN
− ×
SLOPE
1 1 1
)
MIN
.
)
for ensur-
CS
signal diode D11, 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 R9 such that the input bias current of
the current-sense comparators does not add consider-
able error to the current-sense signal. The value of R10
for the slope compensation is given by the equation:
The MAX16809 features a 16-channel, constant-current
LED driver, with each channel capable of sinking up to
55mA of LED current. The LED strings are connected
between VLED and the constant-current-sink outputs to
drive regulated current through LED strings. The cur-
rent through all 16 channels is controlled through resis-
tor (R1) from the SET pin to ground. The MAX16809 EV
kit sets the current through each string at 40mA and the
maximum LED supply voltage to 33V. The MAX16809
EV kit drives LED strings with a total forward voltage of
up to 32V.
A 4-wire serial interface with four inputs (DIN, CLK, LE,
and OE) individually control the constant-current out-
puts. In the MAX16809 EV kit, a 50kHz clock signal,
generated by U4A, clocks 16 1s into the internal shift
register by tying DIN and LE to 5V. The clock-generation
circuit can be avoided if a microcontroller provides the
function.
The output enable (OE) can provide PWM dimming. An
inverted PWM signal, generated by the inverter U4B, is
necessary to drive the OE pin. When the PWM signal is
low (LED drivers off), it also influences the feedback
with the network formed by R6 and D12. See the
Adaptive LED Supply Voltage Control section for more
details.
If an inverted PWM signal is available, use the circuit
shown in Figure1 to drive the OE input and feedback
network.
R
10
VR
=
SLOPE
⎛
⎜
⎝
VC
VR
SLOPE
SLOPE
=
1 7 .
−
×
1
F
⎞
⎟ ×
⎠
SW
R
BE
9
of Q1, almost
LED Driver
SW
is the