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fsw = 500 kHz
Nomin Vin = 48 V
Max Vin = 48 V
VinRipple = 0.5V
Vout = 36.6 V
Rd = ???
Iled = 0.45 A
LED I ripple = 0.05 A
L I ripple = 0.4 A
UVLO turn on = 10 V
UVLO VHys = 1 V
Eff = 0.95
# Roff
Take Coff = 1 nF
−(1−(36.6÷(0.95×48))
--------------------
1nF+20pF×500kHz×ln(1-(1.24/36.6))
= 11.2kΩ
Closest E96 is 11 kΩ
This makes toff
-(1nF+20pF)*11kΩ*ln(1-(1.24V/36.6V))
= 387 ns
And fsw
= (1-(36.6V/(0.95*48)))/387ns = 510 kHz
+-------------------
| Coff = 1nF
| Roff = 11 kΩ
+-------------------
# Inductor ripple
L = (36.6V*387ns)/.4 = 35 µH
Closest E12 inductor is 33 µH
So really,
ILRipple = 36.6V * 387 ns / 33 µH = 429 mA
+-------------------
| L= 36 µH
+-------------------
# Average LED current
ILmax = ILED + 1/2*ILripple
= .45 + 1/2*.429 = 0.665 A
Take Vadj = 1.24 V
Rsense = 1.24/(5*ILmax)
= 1.24/(5*.665) = 0.373 Ω
Closest E96 is... 0.373 Ω
ILed = 1.24/(5*0.373) - (ILripple/2)
= 0.45 A
+------------------
| Rsense = 0.373 Ω
+------------------
# Output capacitance
Rd is about 10 Ω for this part
Zc = (10Ω*ILEDRipple)/(ILripple-ILEDRipple)
= 1.32 Ω
CoMIN = 1/(2π×fsw×Zc)
= 236 nF
+------------------
| Co = CoMin * 1.75 = 413 nF
| = 500 nF (let's round)
+------------------
# Input capacitance
ton = 1/fsw - toff
= 1/510kHz - 387ns
= 1.6 µs
CinMIN = 0.45A*1.6µs/0.5V = 1.44 µF
Multiply by 2 for safety, 2.8 µF
2.7 µF is the closest in the E12.
+------------------
| Cin = 2.7 µF
+------------------
Check that IinRMS is sane:
IinRMS = Iled*fsw*sqrt(ton*toff)
= 181 mA
# PFET
Minimum voltage rating 48 V
Minimum current rating = Iled * (Vo/(Vin*η)
= .45 * (36.6/(48*0.95)
= 361 mA
Chosen part:
ZXMP7A17KTC
Rds(on) = 0.16 Ω
Continuous drain I = 5.7 A
Vds = 70 V
Qg = 9.6 nC
ItRMS = ILed*sqrt(36.6V/(48V*0.95) * (1+1/12*(LIripple/Iled)²))
= 0.45 * sqrt (36.6V/(48V*0.95) * (1+1/12*(.429/.45)²))
= 434 mA
Pt = ItRMS² * Rds(on)
= .434² * .16
= 69 mW
# Diode
Diodes has to be good for 48 V
Id = (1-(36.6/(48*η)))*Iled
= 124 mA
Chosen diode:
B1100-13-F
Vf = 0.79 V
Vrrm = 100 V
If = 1 A
Ifsm = 30 A
Pd = Vd * Id = 0.124 A * .79
= 98 mW
So a 2.5 °C rise, well good
# UVLO
Copy UVLO from datasheet.
Ruv2 = 49.9 kΩ
Rub1 = 6.98 kΩ
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