A8518 Datasheet, PDF (28/30 Pages) Allegro MicroSystems – Fully integrated 42 V MOSFET for boost converter

A8518 and A8518-1
Wide Input Voltage Range, High-Efficiency,
Fault-Tolerant LED Driver
C_IN =
ΔI_L(used)
8 × f_SW × ΔV_IN
C_IN =
0.375 A
8 × 2 MHz × 0.1 V
= 0.234 µF
C = _IN(rms)
I × ^Δ_{II OUT}
L(used)
IN(max)
(1 – D_MAX )× 12
= 0.1 A
C = _IN(rms)
0.240
A
×
0.375 A
1.06 A
(1 – 0.75) × 12
= 0.1 A
A good ceramic input capacitor with ratings of 50 V / 2.2 µF or
50 V / 4.7 µF will suffice for this application.
Vendor
Murata
Murata
Value
4.7 µF / 50 V
2.2 µF / 50 V
Part Number
GRM32ER71H475KA88L
GRM31CR71H225KA88L
If long wires are used for the input, it is necessary to use a much
larger input capacitor. A larger input capacitor is also required to
have stable input voltage during line transients. Combinations of
aluminum electrolytic and ceramic capacitors can be used.
Step 8: Choosing the input disconnect switch components.
Set the input disconnect current limit to 4.25 A.
0.11 V
R=
= 0.0259 Ω
SC _{4.25 A}
The chosen R_SC is 0.024 Ω. The trip point voltage needs to be:
V_SC = 4.25 A× 0.024 Ω = 0.102 V
V – V _VSENSE(trip)
SC
R = _adj
I_adj
R_adj =
0.11 V – 0.102 V
21.5 µA
= 372 Ω
A value of 383 Ω was chosen for this design. The disconnect
switch Q1 works as on or off. Therefore, the R_adj value is not
critical.
For the input disconnect switch, an AO4421 6.2 A / 60 V P-chan-
nel MOSFET is selected.
To achieve proper operation at low dimming ratios, connect an
RC filter to the VOUT pin. Use R = 10 kΩ and C = 47 pF.
V_IN = (4.5 to 40)V
4.7 µF
V_C
10 kW
*optional
10 µH
D1
0.024 W
Q1
383 W
GATE
VSENSE
VIN
VDD
SW
VOUT
OVP
V_OUT > V_IN
158 kW
10 µF 10 µF
1 µF
FAULT
EN/PWM
APWM
ISET
11.8 kW
LED1
LED2
COMP
AGND PGND
100 pF
169 W
0.068 mF
Figure 35: Schematic Showing Calculated Values from the Design Example Above
Allegro MicroSystems, LLC
28
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com