Datasheet

MAX16932/MAX16933
2.2MHz, 36V, Dual Buck
with 20µA Quiescent Current
15Maxim Integrated
value (L), inductor saturation current (I
SAT
), and DC
resistance (R
DCR
). To determine the optimum induc-
tance, knowing the typical duty cycle (D) is important.
= =
−+
OUT OUT
IN IN OUT DS(ON) DCR
VV
D OR D
V V I (R R )
if the R
DCR
of the inductor and R
DS(ON)
of the MOSFET
are available with V
IN
= (V
BAT
- V
DIODE
). All values
should be typical to optimize the design for normal
operation.
Inductance
The exact inductor value is not critical and can be
adjusted in order to make trade-offs among size, cost,
efficiency, and transient response requirements.
• Lower inductor values increase LIR, which minimizes
size and cost and improves transient response at the
cost of reduced efficiency due to higher peak currents.
• Higher inductance values decrease LIR, which
increases efficiency by reducing the RMS current at
the cost of requiring larger output capacitors to meet
load-transient specifications.
The ratio of the inductor peak-to-peak AC current to DC
average current (LIR) must be selected first. A good
initial value is a 30% peak-to-peak ripple current to aver-
age-current ratio (LIR = 0.3). The switching frequency,
input voltage, output voltage, and selected LIR then
determine the inductor value as follows:
IN OUT
SW OUT
(V V )xD
L[µH]
f [MHz]x I x LIR
−
=
where V
IN
, V
OUT
, and I
OUT
are typical values (so that
efficiency is optimum for typical conditions).
Peak Inductor Current
Inductors are rated for maximum saturation current. The
maximum inductor current equals the maximum load cur-
rent in addition to half of the peak-to-peak ripple current:
∆
= +
INDUCTOR
PEAK LOAD(MAX)
I
II
2
For the selected inductance value, the actual peak-to-peak
inductor ripple current (DI
INDUCTOR
) is calculated as:
−
∆=
OUT IN OUT
INDUCTOR
IN SW
V(VV)
I
V xf xL
where DI
INDUCTOR
is in mA, L is in µH, and f
SW
is in kHz.
Once the peak current and the inductance are known,
the inductor can be selected. The saturation current
should be larger than I
PEAK
or at least in a range where
the inductance does not degrade significantly. The
MOSFETs are required to handle the same range of cur-
rent without dissipating too much power.
MOSFET Selection in
Buck Converters
Each step-down controller drives two external logic-level
n-channel MOSFETs as the circuit switch elements. The
key selection parameters to choose these MOSFETs
include the items in the following sections.
Threshold Voltage
All four n-channel MOSFETs must be a logic-level type
with guaranteed on-resistance specifications at V
GS
=
4.5V. If the internal regulator is bypassed (for example:
V
EXTVCC
= 3.3V), then the n-channel MOSFETS should
be chosen to have guaranteed on-resistance at that
gate-to-source voltage.
Maximum Drain-to-Source Voltage (V
DS(MAX)
)
All MOSFETs must be chosen with an appropriate V
DS
rating to handle all V
IN
voltage conditions.