Rainbow Electronics MAX15022 User Manual
Page 15
MAX15022
Dual, 4A/2A, 4MHz, Step-Down DC-DC
Regulator with Dual LDO Controllers
______________________________________________________________________________________
15
Effective Input-Voltage Range
Although the MAX15022’s regulators can operate from
input supplies ranging from 2.5V to 5.5V, the input-volt-
age range can be effectively limited by the
MAX15022’s duty-cycle limitations for a given output
voltage (V
OUT_
). The maximum input voltage
(V
PVIN_MAX
) can be effectively limited by the control-
lable minimum on-time (t
ON(MIN)
):
where t
ON(MIN)
is 0.06μs (typ).
The minimum input voltage (V
PVIN_MIN
) can be effec-
tively limited by the maximum controllable duty cycle
and is calculated using the following equation:
where V
OUT_
is the regulator output voltage and
t
OFF(MIN)
is the 0.06μs (typ) controllable off-time.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX15022: inductance value (L),
peak inductor current (I
PEAK
), and inductor saturation
current (I
SAT
). The minimum required inductance is a
function of operating frequency, input-to-output voltage
differential, and the peak-to-peak inductor current
(
ΔI
P-P
). Higher
ΔI
P-P
allows for a lower inductor value. A
lower inductance minimizes size and cost and
improves large-signal and transient response.
However, efficiency is reduced due to higher peak cur-
rents and higher peak-to-peak output-voltage ripple for
the same output capacitor. A higher inductance
increases efficiency by reducing the ripple current;
however, resistive losses due to extra wire turns can
exceed the benefit gained from lower ripple current lev-
els especially when the inductance is increased without
also allowing for larger inductor dimensions. Choose
the inductor’s peak-to-peak current,
ΔI
P-P,
in the range
of 20% to 50% of the full load current; as a rule of
thumb 30% is typical.
Calculate the inductance, L, using the following equation:
where V
PVIN_
is the input supply voltage, V
OUT_
is the
regulator output voltage, and f
SW
is the switching fre-
quency. Use typical values for V
PVIN_
and V
OUT_
so
that efficiency is optimum for typical conditions. The
switching frequency (f
SW
) is programmable between
500kHz and 4MHz (see the
Oscillator
section).
The peak-to-peak inductor current (
ΔI
P-P
), which
reflects the peak-to-peak output ripple, is largest at the
maximum input voltage. See the
Output-Capacitor
Selection
section to verify that the worst-case output
current ripple is acceptable.
Select an inductor with a saturation current, I
SAT
, high-
er than the maximum peak current to avoid runaway
current during continuous output short-circuit condi-
tions. Also, confirm that the inductor’s thermal perfor-
mances and projected temperature rise above ambient
does not exceed its thermal capacity. Many inductor
manufacturers provide bias/load current versus tem-
perature rise performance curves (or similar) to obtain
this information.
Input-Capacitor Selection
The discontinuous input current of the buck converter
causes large input ripple currents and therefore the
input capacitor must be carefully chosen to withstand
the input ripple current and keep the input-voltage rip-
ple within design requirements.
The input-voltage ripple is comprised of
ΔV
Q
(caused
by the capacitor discharge) and
ΔV
ESR
(caused by the
ESR of the input capacitor). The total voltage ripple is
the sum of
ΔV
Q
and
ΔV
ESR
which peaks at the end of
the on-cycle. Calculate the required input capacitance
and ESR for a specified ripple using the following equa-
tions:
I
LOAD(MAX)
is the maximum output current,
ΔI
P-P
is the
peak-to-peak inductor current, and V
PVIN_
is the input
supply voltage, V
OUT_
is the regulator output voltage,
and f
SW
is the switching frequency.
ESR
V
[mV]
I
I
2
[A]
C
I
[A]
V
[V]
V
[V]
V [V] f
[MHz]
I
[A]
V
V
[V] V
[V]
V
[V] f
[MHz] L
ESR
LOAD(MAX)
P P
PVIN_
LOAD(MAX)
OUT_
PVIN_
Q
SW
P P
PVIN_
OUT_
OUT_
PVIN_
SW
[
]
[
]
[
]
m
F
H
Ω
Δ
Δ
Δ
Δ
=
+
⎛
⎝⎜
⎞
⎠⎟
=
Ч
⎛
⎝
⎜
⎞
⎠
⎟
Ч
=
−
(
)
Ч
Ч
Ч
−
−
μ
μ
L H
I
A
P P
[
]
[ ]
μ =
Ч
−
Ч
Ч
−
V
[V] (V
[V] V
[V])
V
[V] f
[MHz]
OUT_
PVIN_
OUT_
PVIN_
SW
Δ
V
[V]
V
[V]
1 (t
[ s] f
[MHz])
PVIN_MIN
OUT_
OFF(MIN)
SW
≥
−
×
μ
V
[V]
V
[V]
t
[ s] f
[MHz]
PVIN_MAX
OUT_
ON(MIN)
SW
≤
×
μ