MAX16834 Datasheet, PDF (14/23 Pages) Maxim Integrated Products – High-Power LED Driver with Integrated High-Side LED Current Sense and PWM Dimming MOSFET Driver

High-Power LED Driver with Integrated High-Side LED
Current Sense and PWM Dimming MOSFET Driver
configuration, the input current is the inductor current
minus the LED current. But for both configurations, the
ripple current that the input filter capacitor has to sup-
ply is the same as the inductor ripple current with the
condition that the output filter capacitor should be con-
nected to ground for buck-boost configuration. This
reduces the size of the input capacitor, as the inductor
current is continuous with maximum ±30% ripple.
Neglecting the effect of LED current ripple, the calcula-
tion of the input capacitor for boost as well as buck-
boost configurations is the same.
Neglecting the effect of the ESL, the ESR, and the bulk
capacitance at the input contributes to the input voltage
ripple. For simplicity, assume that the contribution from
the ESR and the bulk capacitance is equal. This allows
50% of the ripple for the bulk capacitance. The capaci-
tance is given by:
C_IN
≥
4
×
∆I_L
∆V_IN ×
f_SW
where ∆I_L is in amperes, C_IN is in farads, f_SW is in hertz,
and ∆V_IN is in volts. The remaining 50% of allowable
ripple is for the ESR of the output capacitor. Based on
this, the ESR of the input capacitor is given by:
ESR _CIN
<
∆V_IN
∆I_L × 2
where ∆I_L is in amperes, ESR_CIN is in ohms, and ∆V_IN
is in volts.
Use the below equation to calculate the RMS current
rating of the input capacitor:
I_CIN(RMS)
=
∆I_L
23
Slope Compensation
Slope compensation should be added to converters
with peak current-mode control operating in continuous
conduction mode with more than 50% duty cycle to
avoid current loop instability and subharmonic oscilla-
tions. The minimum amount of slope added to the peak
inductor current to stabilize the current control loop is
half of the falling slope of the inductor.
In the MAX16834, the slope compensating ramp is
added to the current-sense signal before it is fed to the
PWM comparator. Connect a capacitor (C2 in the appli-
cation circuit) from SC to ground for slope compensa-
tion. This capacitor is charged with a 100µA current
source and discharged at the beginning of each switch-
ing cycle to generate the slope compensation ramp.
The value of the slope compensation capacitor C2 is
calculated as shown below:
Boost configuration:
C2 =
3 × L × 100 × 10 ^-6
(V_LED - V_INMIN) × R8 × 2
where C2 is in farads, L is the inductance of the induc-
tor L1 in henries, 100µA is the pullup current from SC,
V_LED and V_INMIN are in volts, and R8 is the switch cur-
rent-sense resistor in ohms.
Buck-boost configuration:
C2 = ^{3 × L × 100 × 10 -6}
(V_LED) × R8 × 2
where C2 is in farads, L is the inductance of the induc-
tor L1 in henries, 100µA is the pullup current from SC,
V_LED is in volts, and R8 is the switch current-sense
resistor in ohms.
Selection of Power Semiconductors
Switching MOSFET
The switching MOSFET (Q1) should have a voltage rat-
ing sufficient to withstand the maximum output voltage
together with the diode drop of the rectifier diode D1
and any possible overshoot due to ringing caused by
parasitic inductances and capacitances. Use a
MOSFET with a drain-to-source voltage rating higher
than that calculated by the following equations:
Boost configuration:
V_DS = (V_LED + V_D ) × 1.2
where V_DS is the drain-to-source voltage in volts and
V_D is the forward drop of the rectifier diode D1. The fac-
tor of 1.2 provides a 20% safety margin.
Buck-boost configuration:
V_DS = (V_LED + V_INMAX + V_D ) × 1.2
where V_DS is the drain-to-source voltage in volts and
V_D is the forward drop of the rectifier diode D1. The fac-
tor of 1.2 provides a 20% safety margin.
The continuous drain current rating of the selected
MOSFET, when the case temperature is at +70°C,
should be greater than the value calculated by the fol-
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