MAX16930 Datasheet, PDF (17/29 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
Design Procedure
DC output accuracy specifications in the Electrical
Characteristics table refer to the error comparator’s
Buck Converter Design Procedure
Effective Input Voltage Range in Buck Converters
Although the MAX16930/MAX16931 can operate from
input supplies up to 36V (42V transients) and regulate
down to 1V, the minimum voltage conversion ratio (V_OUT/
V_IN) might be limited by the minimum controllable on-time.
For proper fixed-frequency PWM operation and optimal
threshold, V_{FB_} = 1V (typ). When the inductor conducts
continuously, the devices regulate the peak of the output
ripple, so the actual DC output voltage is lower than the
slope-compensated trip level by 50% of the output ripple
voltage.
In discontinuous conduction mode (skip or STDBY active
and I_OUT < I_LOAD(SKIP)), the devices regulate the valley
efficiency, buck 1 and buck 2 should operate in continu- of the output ripple, so the output voltage has a DC regu-
ous conduction during normal operating conditions. For lation level higher than the error-comparator threshold.
continuous conduction, set the voltage conversion ratio
as follows:
Inductor Selection in Buck Converters
Three key inductor parameters must be specified for
V_OUT
V_IN
> t _ON(MIN) × f_SW
operation with the MAX16930/MAX16931: inductance
value (L), inductor saturation current (I_SAT), and DC
where t_ON(MIN) is 50ns (typ) and f_SW is the switching
resistance (R_DCR). To determine the optimum induc-
tance, knowing the typical duty cycle (D) is important.
frequency in Hz. If the desired voltage conversion does
not meet the above condition, pulse skipping occurs to_=D ^V₌ OUT _{OR D}
V_OUT
decrease the effective duty cycle. Decrease the switching
V_IN
V_IN − I_OUT (R_DS(ON) + R_DCR )
frequency if constant switching frequency is required. The
same is true for the maximum voltage conversion ratio.
The maximum voltage conversion ratio is limited by the
maximum duty cycle (95%).
V_OUT
< 0.95
V_IN − V_DROP
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
where V_DROP = I_OUT (R_ON,HS + R_DCR) is the sum of the
parasitic voltage drops in the high-side path and f_SW is
the programmed switching frequency. During low drop
adjusted in order to make trade-offs among size, cost,
efficiency, and transient response requirements.
• Lower inductor values increase LIR, which minimizes
operation, the devices reduce f_SW to 25% (max) of the
programmed frequency. In practice, the above condition
size and cost and improves transient response at the
cost of reduced efficiency due to higher peak currents.
should be met with adequate margin for good load-tran- _{• Higher inductance values decrease LIR, which}
sient response.
increases efficiency by reducing the RMS current at
Setting the Output Voltage
the cost of requiring larger output capacitors to meet
in Buck Converters
load-transient specifications.
Connect FB1 and FB2 to BIAS to enable the fixed buck _{The ratio of the inductor peak-to-peak AC current to DC}
controller output voltages (5V and 3.3V) set by a preset _{average current (LIR) must be selected first. A good}
internal resistive voltage-divider connected between the
output (OUT_) and AGND. To externally adjust the output
voltage between 1V and 10V, connect a resistive divider
from the output (OUT_) to FB_ to AGND (see the Typical
Operating Circuit. Calculate R_{FB_1} and R_{FB_2} with the fol-
lowing equation:
= R_{FB_1}
R_{FB_2}


V_{OUT _}
V_{FB_}


−

1

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:
L[µH] = ^{(VIN − VOUT )x D}
f_SW [MHz]x I_OUT x LIR
where V_IN, V_OUT, and I_OUT are typical values (so that
where VFB_ = 1V (typ) (see the Electrical Characteristics ^{efficiency is optimum for typical conditions).}
table).
Maxim Integrated
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