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MAX16814
Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
Feedback Compensation
During normal operation, the feedback control loop reg-
For SEPIC and coupled-inductor boost-buck configurations:
ulates the minimum OUT_ voltage to 1V when LED string
currents are enabled during PWM dimming. When LED
currents are off during PWM dimming, the control loop
f P1 =
I LED × D MAX
2 × π × V LED × C OUT
V LED (1 ? D MAX ) 2
f ZRHP =
R COMP =
V LED (1 ? D MAX ) 2
f ZRHP =
R COMP =
turns off the converter and stores the steady-state condi-
tion in the form of capacitor voltages, mainly the output
filter capacitor voltage and compensation capacitor
voltage. For the MAX16814A_ _ and the MAX16814U_
_, when the PWM dimming pulses are less than or equal
to 5 switching clock cycles, the feedback loop regulates
the converter output voltage to 95% of OVP threshold.
The worst-case condition for the feedback loop is when
the LED driver is in normal mode regulating the minimum
OUT_ voltage to 1V. The switching converter small-signal
transfer function has a right-half plane (RHP) zero for
boost configuration if the inductor current is in continuous
conduction mode. The RHP zero adds a 20dB/decade
gain together with a 90 N -phase lag, which is difficult to
compensate.
The worst-case RHP zero frequency (f ZRHP ) is calcu-
lated as follows:
For boost configuration:
2 π × L × I LED
For SEPIC and coupled-inductor boost-buck configura-
tions:
2 π × L × I LED × D MAX
where f ZRHP is in hertz, V LED is in volts, L is the induc-
tance value of L1 in henries, and I LED is in amperes. A
simple way to avoid this zero is to roll off the loop gain
to 0dB at a frequency less than one fifth of the RHP zero
where f P1 is in hertz, V LED is in volts, I LED is in amperes,
and C OUT is in farads.
Compensation components, R COMP and C COMP , per-
form two functions. C COMP introduces a low-frequency
pole that presents a -20dB/decade slope to the loop
gain. R COMP flattens the gain of the error amplifier for
frequencies above the zero formed by R COMP and
C COMP . For compensation, this zero is placed at the
output pole frequency f P1 so that it provides a -20dB/
decade slope for frequencies above f P1 to the combined
modulator and compensator response.
The value of R COMP needed to fix the total loop gain at
f P1 so that the total loop gain crosses 0dB with -20dB/
decade slope at 1/5 the RHP zero frequency is calcu-
lated as follows:
For boost configuration:
f ZRHP × R CS × I LED
5 × f P1 × GM COMP × V LED × (1 ? D MAX )
For SEPIC and coupled-inductor boost-buck configura-
tions:
f ZRHP × R CS × I LED × D MAX
5 × f P1 × GM COMP × V LED × (1 ? D MAX )
where R COMP is the compensation resistor in ohms,
f ZRHP and f P2 are in hertz, R CS is the switch current-
sense resistor in ohms, and GM COMP is the transcon-
ductance of the error amplifier (600 F S).
The value of C COMP is calculated as follows:
frequency with a -20dB/decade slope.
The switching converter small-signal transfer function
also has an output pole. The effective output impedance
C COMP =
1
2 π × R COMP × f Z1
together with the output filter capacitance determines the
output pole frequency f P1 that is calculated as follows:
For boost configuration:
where f Z1 is the compensation zero placed at 1/5 of
the crossover frequency that is, in turn, set at 1/5 of the
f ZRHP .
Maxim Integrated
f P1 =
I LED
2 × π × V LED × C OUT
If the output capacitors do not have low ESR, the ESR
zero frequency may fall within the 0dB crossover fre-
quency. An additional pole may be required to cancel
out this pole placed at the same frequency. This is usu-
ally implemented by connecting a capacitor in parallel
19
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