MAX16833_11 Datasheet, PDF (18/22 Pages) Maxim Integrated Products – High-Voltage HB LED Drivers with Integrated High-Side Current Sense

High-Voltage HB LED Drivers with
Integrated High-Side Current Sense
Boost Configuration
P_SW
=
^{IL AV G}

×
V_LED ²
2
×
C_GD
×
f_SW



×
1


IG_ON
+
1
IG_OFF



Buck-Boost Configuration
P_SW
=
_{IL AV G}

×
(V_LED
+
V_INMAX ) ²
2
×
C_GD
×
f_SW



×
1
_IGON
+
1
IG_OFF



where IGON and IGOFF are the gate currents of the
MOSFET Q1 in amperes when it is turned on and turned
off, respectively, VLED and VINMAX are in volts, ILAVG is
in amperes, fSW is in hertz, and CGD is the gate-to-drain
MOSFET capacitance in farads.
Rectifier Diode
Use a Schottky diode as the rectifier (D1) for fast switch-
ing and to reduce power dissipation. The selected
Schottky diode must have a voltage rating 20% above
the maximum converter output voltage. The maximum
converter output voltage is VLED in boost configuration
and VLED + VINMAX in buck-boost configuration.
The current rating of the diode should be greater than ID
in the following equation:
ID = ILAVG x (1 - DMAX) x 1.5
Dimming MOSFET
Select a dimming MOSFET (Q2) with continuous current
rating at the operating temperature higher than the LED
current by 30%. The drain-to-source voltage rating of the
dimming MOSFET must be higher than VLED by 20%.
Feedback Compensation
The LED current control loop comprising the switching
converter, the LED current amplifier, and the error ampli-
fier should be compensated for stable control of the LED
current. The switching converter small-signal transfer
function has a right-half-plane (RHP) zero for both boost
and buck-boost configurations as the inductor current
is in continuous conduction mode. The RHP zero adds
a 20dB/decade gain together with a 90-degree 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 1/5 the RHP zero frequency with a
-20dB/decade slope.
The worst-case RHP zero frequency (fZRHP) is calcu-
lated as follows:
Boost Configuration
f _ZRHP
=
V_LED × (1- D_MAX )²
2π × L ×I_LED
Buck-Boost Configuration
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.
The switching converter small-signal transfer function
also has an output pole for both boost and buck-boost
configurations. The effective output impedance that
determines the output pole frequency together with the
output filter capacitance is calculated as follows:
Boost Configuration
R _OUT
=
(R_LED + R7) × V_LED
(R_LED + R7) × I_LED + V_LED
Buck-Boost Configuration
R_OUT
=
(R_LED + R7) × V_LED
(R_LED + R7) × I_LED × D_MAX +
V_LED
where RLED is the dynamic impedance of the LED string
at the operating current in ohms, R7 is the LED current-
sense resistor in ohms, VLED is in volts, and ILED is in
amperes.
The output pole frequency for both boost and buck-
boost configurations is calculated as below:
f_P2 =
1
2π × C_OUT × R_OUT
where fP2 is in hertz, COUT is the output filter capaci-
tance in farads, and ROUT is the effective output imped-
ance in ohms calculated above.
The feedback loop compensation is done by connecting
resistor R10 and capacitor C4 in series from the COMP
pin to GND. R10 is chosen to set the high-frequency gain
of the integrator to set the crossover frequency at fZRHP/5
and C4 is chosen to set the integrator zero frequency
to maintain loop stability. For optimum performance,
choose the components using the following equations:
R10 =
2 × f_ZRHP × R4
F_C × (1 − D_MAX ) × R7 × 6.15 × GM_COMP
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