A4450 Datasheet, PDF (20/34 Pages) Allegro MicroSystems – Buck-Boost Controller with Integrated Buck MOSFET

A4450
Buck-Boost Controller
with Integrated Buck MOSFET
Output Inductor (L_O)
In Buck mode, the peak inductor current is:
For the peak current-mode control, the priority to consider when
selecting the inductor is to prevent the subharmonic oscillations
I_PEAK = I_OUT +
ΔI_L
2
(10)
when the duty cycle is near or above 50%. To prevent the subhar-
monic oscillations, the theorectical slope compensation ramp S_{E where}
should be at least 50% of the down slope of the inductor current.
In the A4450, the inductor value L_O is suggested to start with the
equation below, because A4450 will also operate in Buck-Boost
mode:
ΔI_L =
V_OUT × (V_IN(MAX) – V_OUT )
V_IN(MAX) × f_SW × L_O
( ) ^{LO ≥ 1.5 × max}
V_OUT
– V_IN(MIN)
S_E
,
0.5 × V_OUT
S_E
(8)
where V_OUT is the output voltage, V_IN(MIN) is the minimum input
voltage, S_E is in A/µs, which scales with the switching frequency
and can be estimated by interpolating from Electrical Characteris-
tics table, and L_O is in µH.
Ideally, the inductor should not saturate at the highest pulse-by-
pulse current limit. This may be too costly. At the very least, the
inductor should not saturate at the peak operating current.
In Buck-Boost mode, the peak inductor current is calculated as:
I_PEAK =
I_OUT +
ΔI_L
2
× (1 – D_Buck)
1 – D_BOOST(MAX)
(9)
where
{ ^{ΔIL = max}
V_IN(MIN) × D_{Boost(MAX) ,} V_IN(MIN) × D_Boost(MAX)
f_SW × L_O
f_SW × L_O
} ⁺
V_IN(MIN) – V_OUT
f_SW × L_O
× (D_Buck – D_Boost(MAX))
After an inductor is chosen, it should be tested during output
short-circuit conditions. The inductor current should be moni-
tored using a current probe. A good design should ensure neither
the inductor nor the regulator are damaged when the output is
shorted to ground at the highest expected ambient temperature.
Buck Diode (D₁) and Boost Diode (D₂)
Schottky diodes with proper ratings should be chosen for the
buck diode (D₁) and boost diode (D₂), because of their low
forward voltage drop and fast reverse recovery time. The key
parameters in D₁ and D₂ selection are the average forward current
I_f(AVG) and the DC reverse voltage.
The boost diode D₂ should be greater than V_OUT with some
margin. The average forward current rating of the boost diode
should be above the full load current, and the peak current should
be above the peak inductor current which is calculated in previous
Output Inductor section.
The buck diode D₁ must be able to withstand the input voltage
when the high-side buck switch is on. Therefore, the reverse
voltage rating should be greater than the maximum expected V_IN
with some margin.
When the buck switch is off, the buck diode D₁ must conduct the
output current. The average forward current rating of the buck
diode should be above the full load current.
D_Boost(MAX) = 1 –
V_IN(MIN) × 1.844
R_NG
D_Buck(MAX) = (1 – D_Boost(MAX)) ×
V_OUT
V_IN(MIN)
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