MAX16814_15 Datasheet, PDF (19/25 Pages) Maxim Integrated Products – Integrated, 4-Channel, High-Brightness LED Driver with High-Voltage DC-DC Controller

MAX16814
Integrated, 4-Channel, High-Brightness LED
Driver with High-Voltage DC-DC Controller
Feedback Compensation
During normal operation, the feedback control loop reg-
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
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 90N-phase lag, which is difficult to
compensate.
The worst-case RHP zero frequency (fZRHP) is
calculated as follows:
For boost configuration:
f_ZRHP =
V_LED(1− D_MAX )²
2π × L ×I_LED
For SEPIC and coupled-inductor
configurations:
boost-buck
f_ZRHP =
V_LED(1− D_MAX )²
2π × L ×I_LED × D_MAX
where fZRHP is in hertz, VLED is in volts, L is the induc-
tance value of L1 in henries, and ILED 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
frequency with a -20dB/decade slope.
The switching converter small-signal transfer function
also has an output pole. The effective output impedance
together with the output filter capacitance determines the
output pole frequency fP1 that is calculated as follows:
For boost configuration:
f_P1
=
2
×
π
×
I_LED
V_LED ×
C _OUT
For SEPIC and coupled-inductor boost-buck configurations:
f_P1
=
2
×
I_LED × D_MAX
π × V_LED × C_OUT
where fP1 is in hertz, VLED is in volts, ILED is in amperes,
and COUT is in farads.
Compensation components (RCOMP and CCOMP)
perform two functions. CCOMP introduces a low-
frequency pole that presents a -20dB/decade slope
to the loop gain. RCOMP flattens the gain of the error
amplifier for frequencies above the zero formed by
RCOMP and CCOMP. For compensation, this zero is
placed at the output pole frequency fP1 so that it pro-
vides a -20dB/decade slope for frequencies above fP1
to the combined modulator and compensator response.
The value of RCOMP needed to fix the total loop gain
at fP1 so that the total loop gain crosses 0dB with
-20dB/decade slope at 1/5 the RHP zero frequency is
calculated as follows:
For boost configuration:
R _COMP
=
5×
f_P1 ×
f_ZRHP × R_CS ×I_LED
GM_COMP × V_LED × (1− D_MAX )
For SEPIC and coupled-inductor boost-buck
configurations:
R _COMP
=
5×
f_ZRHP × R_CS
f_P1 × GM_COMP
×I_LED × D_MAX
× V_LED × (1− D_MAX )
where RCOMP is the compensation resistor in
ohms, fZRHP and fP2 are in hertz, RCS is the switch
current-sense resistor in ohms, and GMCOMP is the
transconductance of the error amplifier (600FS).
The value of CCOMP is calculated as follows:
C_COMP =
1
2π × R_COMP × f_Z1
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.
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
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