MAX20070 datasheet(31/34 Pages) MAXIM

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Feedback Compensation

During normal operation, the feedback control loop

regulates the minimum OUT_ voltage to 0.75V 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

condition in the form of capacitor voltages, mainly the

output filter-capacitor voltage and compensation-capacitor

voltage. When the PWM dimming pulses are less than 24

switching clock cycles, the feedback loop regulates the

converter output voltage to 95% of the 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 0.75V. 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° phase lag, which

is difficult to compensate. The worst-case RHP zero

frequency (fZRHP) is calculated in the following equations.

For boost configuration:

LED

MAX

ZRHP

LED

(1 D

)

−

π× ×

For SEPIC configuration:

LED

MAX

ZRHP

LED

MAX

(1 D

)

−

π× ×

where fZRHP is in hertz, VLED is in volts, Lis the inductance

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 1/5 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 in the

following equations.

For boost configuration:

LED

OUT

× π× ×

For SEPIC configuration:

LED

MAX

LED

OUT

× π× ×

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 it provides

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 the total loop gain crosses 0dB with -20dB/ decade

slope at 1/5 the RHP zero frequency, is calculated in the

following equations.

For boost configuration:

ZRHP

LED

COMP

LED

MAX

5 f

(1 D

)

××

× −

For the SEPIC configuration:

ZRHP

LED

MAX

COMP

LED

MAX

5 f

(1 D

)

××

× −

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 (600μS).

The value of RCOMP is calculated as follows:

COMP

π×

where fZ1 is the compensation zero placed at 1/5 of the

crossover frequency, which is, in turn, set at 1/5 of the

fZRHP. If the output capacitors do not have low ESR, the

ESR zero frequency may fall within the 0dB crossover

frequency. An additional pole may be required to cancel

out this pole placed at the same frequency. This is usually

implemented by connecting a capacitor in parallel with

CCOMP and RCOMP.

MAX20070

Integrated TFT Power Supply and

LED Backlight Driver

www.maximintegrated.com

Maxim Integrated │ 31

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