MAX16833 Datasheet, PDF (17/22 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
where ILED is in amperes, COUT is in farads, fSW is in
hertz, and VOUTRIPPLE is in volts. The remaining 50% of
allowable ripple is for the ESR of the output capacitor.
Based on this, the ESR of the output capacitor is given by:
ESR _COUT
<
V_OUTRIPPLE
(IL_P × 2)
(Ω)
where ILP is the peak-inductor current in amperes. Use
the equation below to calculate the RMS current rating of
the output capacitor:
I_COUT(RMS) = IL _AVG2 D_MAX (1 − D_MAX )
Input Capacitor
The input-filter capacitor bypasses the ripple current
drawn by the converter and reduces the amplitude of
high-frequency current conducted to the input supply.
The ESR, ESL, and the bulk capacitance of the input
capacitor contribute to the input ripple. Use a low-ESR
input capacitor that can handle the maximum input RMS
ripple current from the converter. For the boost con-
figuration, the input current is the same as the inductor
current. For buck-boost configuration, the input current
is the inductor current minus the LED current. However,
for both configurations, the ripple current that the input
filter capacitor has to supply is the same as the induc-
tor ripple current with the condition that the output filter
capacitor should be connected to ground for buck-boost
configuration. This reduces the size of the input capaci-
tor, as the input current is continuous with maximum
QDIL/2. Neglecting the effect of LED current ripple, the
calculation 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 contribute to the input-voltage
ripple. For simplicity, assume that the contributions from
the ESR and the bulk capacitance are 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 DIL is in amperes, CIN is in farads, fSW is in hertz,
and DVIN is in volts. The remaining 50% of allowable
ripple is for the ESR of the input capacitor. Based on this,
the ESR of the input capacitor is given by:
ESR _CIN
<
∆V_IN
∆I_L × 2
where DIL is in amperes, ESRCIN is in ohms, and DVIN
is in volts. Use the equation below to calculate the RMS
current rating of the input capacitor:
I_CIN(RMS)
=
∆I_L
23
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 rectifier diode D1 and
any possible overshoot due to ringing caused by parasit-
ic inductances and capacitances. Use a MOSFET with a
drain-to-source voltage rating higher than that calculated
by the following equations.
Boost Configuration
VDS = (VLED + VD) x 1.2
where VDS is the drain-to-source voltage in volts and VD
is the forward drop of rectifier diode D1. The factor of 1.2
provides a 20% safety margin.
Buck-Boost Configuration
VDS = (VLED + VINMAX + VD) x 1.2
where VDS is the drain-to-source voltage in volts and VD
is the forward drop of rectifier diode D1. The factor of 1.2
provides a 20% safety margin.
The RMS current rating of the switching MOSFET Q1 is cal-
culated as follows for boost and buck-boost configurations:
I_DRMS = 1.3 × ( (IL _AVG)² × D_MAX )
where IDRMS is the MOSFET Q1’s drain RMS current in
amperes.
The MOSFET Q1 dissipates power due to both switching
losses, as well as conduction losses. The conduction
losses in the MOSFET are calculated as follows:
PCOND = (ILAVG)² x DMAX x RDSON
where RDSON is the on-resistance of Q1 in ohms, PCOND
is in watts, and ILAVG is in amperes. Use the follow-
ing equations to calculate the switching losses in the
MOSFET.
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