MPQ3426- 6A, 35V BOOST CONVERTER WITH PROGRAMMABLE SWITCHING FREQUENCY AND UVLO AEC-Q100 QUALIFIED
voltage rises. When the SS voltage reaches
technique, use either tantalum- or electrolytic-
type capacitors for the larger capacitor. Place all
ceramic capacitors close to the MPQ3426.
250mV, the MPQ3426 starts switching at 1/5 the
programmed frequency (frequency fold-back
mode). At 800mV the switching frequency rises
to the programmed value. The soft-start ends
when the SS voltage reaches 2.5V. This limits
the inductor current at start-up, forcing the input
current to rise slowly to the required current to
regulate the output voltage.
Selecting the Output Capacitor
The output capacitor maintains the DC output
voltage. For best results, use low-ESR capacitors
to minimize the output voltage ripple. The output
capacitor’s characteristics also affect regulatory
control system’s stability. For best results, use
ceramic, tantalum, or low-ESR electrolytic
capacitors. For ceramic capacitors, the
capacitance dominates the impedance at the
switching frequency, and so the output voltage
ripple is mostly independent of the ESR. The
output voltage ripple is estimated as
The soft-start period is determined by the
equation:
CSS 109 2.5V
tSS
6A
Where CSS (nF) is the soft-start capacitor from
SS to GND, and tSS is the soft-start period.
VIN
1
Setting the Output Voltage
VOUT
VOUT connects to the top of a resistor divider (R2
and R3); the resistor divider’s tap connects to the
FB pin. The feedback voltage is typically 1.225V.
The output voltage is then:
VRIPPLE ILOAD
:
COUT fSW
Where VRIPPLE is the output ripple voltage, VIN and
VOUT are the DC input and output voltages,
respectively, ILOAD is the load current, fSW is the
switching frequency, and COUT is the value of the
output capacitor.
R2
R3
VOUT V 1
FB
Where:
R2 is the top feedback resistor
For tantalum or low-ESR electrolytic capacitors,
the ESR dominates the impedance at the
switching frequency, and so the output ripple is:
R3 is the bottom feedback resistor
VFB is the feedback reference voltage
(typically 1.225V)
VIN
To increase efficiency, use ≥10kꢀ feedback
resistors.
1
VOUT
ILOAD RESR VOUT
VRIPPLE ILOAD
COUT fSW
VIN
Selecting the Input Capacitor
The input requires a capacitor to supply the AC
ripple current to the inductor, while limiting noise
at the input source. Use a low-ESR capacitor
with a value >4.7µF to minimize the IC noise.
Ceramic capacitors are preferred, but tantalum or
low-ESR electrolytic capacitors can also suffice.
However since it absorbs the input switching
current it requires an adequate ripple current
rating. Use a capacitor with an RMS current
rating greater than the inductor ripple current.
Where RESR is the equivalent series resistance of
the output capacitors.
Choose an output capacitor that satisfies the
output ripple and load transient requirements of
the design. A 4.7µF-to-22µF ceramic capacitor is
suitable for most applications.
Selecting the Inductor
The inductor forces the output voltage higher
than the input voltage. A larger inductor value
results in less ripple current and reduces the
peak inductor current; this reduces the stress on
the internal N-channel switch. However, a larger-
value inductor is physically larger, has a higher
series resistance, and/or lower saturation current.
To ensure stable operation, place the input
capacitor as close to the IC as possible. As an
alternative, place a small, high-quality ceramic
0.1µF capacitor close to the IC and place the
larger capacitor further away. If using the latter
MPQ3426 Rev.1.01
7/19/2017
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