MAX16930 Datasheet, PDF (18/29 Pages) Maxim Integrated Products – 2MHz, 36V, Dual Buck with Preboost and 20μA Quiescent Current

MAX16930/MAX16931
2MHz, 36V, Dual Buck with Preboost and
20µA Quiescent Current
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:
= I_PEAK
I_{LOAD (MAX )}
+
∆I_INDUCTOR
2
For the selected inductance value, the actual peak-to-peak
inductor ripple current (DI_INDUCTOR) is calculated as:
∆I_INDUCTOR
= ^{VOUT (VIN − VOUT )}
V_IN x f_SW x L
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 current 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.
Current Capability
The n-channel MOSFETs must deliver the average cur-
rent to the load and the peak current during switching.
Choose MOSFETs with the appropriate average current
at V_GS = 4.5V or V_GS = V_EXTVCC when the internal linear
regulator is bypassed. For load currents below approxi-
mately 3A, dual MOSFETs in a single package can be
an economical solution. To reduce switching noise for
smaller MOSFETs, use a series resistor in the BST_ path
and additional gate capacitance. Contact the factory for
guidance using gate resistors.
Current-Sense Measurement
For the best current-sense accuracy and overcur-
rent protection, use a ±1% tolerance current-sense
resistor between the inductor and output as shown in
Figure 1A. This configuration constantly monitors the
inductor current, allowing accurate current-limit pro-
tection. Use low-inductance current-sense resistors
for accurate measurement.
Alternatively, high-power applications that do not require
highly accurate current-limit protection can reduce the
overall power dissipation by connecting a series RC
circuit across the inductor (Figure 1B) with an equivalent
time constant:
and:
R _CSHL
=



R₂
R₁ + R
2

 ^R_DCR

= R_DCR
L
C_EQ
1


R₁
+
1
R2


where R_CSHL is the required current-sense resistor and
R_DCR is the inductor’s series DC resistor. Use the induc-
tance and R_DCR values provided by the inductor
manufacturer.
Carefully observe the PCB layout guidelines to ensure
the noise and DC errors do no corrupt the differential
current-sense signals seen by CS_ and OUT_. Place
the sense resistor close to the devices with short, direct
traces, making a Kelvin-sense connection to the current-
sense resistor.
Input Capacitor in Buck Converters
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 ripple within
design requirements. The 180° ripple phase operation
increases the frequency of the input capacitor ripple
current to twice the individual converter switching fre-
quency. When using ripple phasing, the worst-case input
capacitor ripple current is when the converter with the
highest output current is on.
Maxim Integrated
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