MBRS130 Ver la hoja de datos (PDF) - Monolithic Power Systems
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MBRS130 Datasheet PDF : 11 Pages
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TM
MP1570 – 3A, 23V, 340KHz SYNCHRONOUS RECTIFIED, STEP-DOWN CONVERTER
OPERATION
FUNCTIONAL DESCRIPTION
The MP1570 is a synchronous rectified,
current-mode, step-down regulator. It regulates
input voltages from 4.75V to 23V down to an
output voltage as low as 1.230V, and supplies
up to 3A of load current.
The MP1570 uses current-mode control to
regulate the output voltage. The output voltage
is measured at FB through a resistive voltage
divider and amplified through the internal
transconductance error amplifier. The voltage at
COMP pin is compared to the switch current
measured internally to control the output
voltage.
The converter uses internal N-Channel
MOSFET switches to step-down the input
voltage to the regulated output voltage. Since
the high side MOSFET requires a gate voltage
greater than the input voltage, a boost capacitor
connected between SW and BS is needed to
drive the high side gate. The boost capacitor is
charged from the internal 5V rail when SW is low.
When the MP1570 FB pin exceeds 20% of the
nominal regulation voltage of 1.230V, the over
voltage comparator is tripped and latched; the
COMP pin and the SS pin are discharged to
GND, forcing the high-side switch off. Latch
cannot be cleared unless the EN or IN pin is
reset.
Following discharge, the MP1570 actively
recycles the energy stored in the output
capacitor. Initially the low-side synchronous
rectifier turns on. Once the internal, negative
900mA current limit is reached, the low-side
switch turns off, forcing inductor current to flow
through the high-side switch body diode. The
inductor current is recycled back into the input
as an energy saving feature. This cycle
continues until the output voltage is discharged
below 10% of the initial regulation voltage
(0.123V at FB), at which time the low-side
switch turns off.
APPLICATIONS INFORMATION
COMPONENT SELECTION
Setting the Output Voltage
The output voltage is set using a resistive
voltage divider from the output voltage to FB
pin. The voltage divider divides the output
voltage down to the feedback voltage by the
ratio:
VFB
=
VOUT
R2
R1+ R2
Thus the output voltage is:
VOUT
= 1.23 × R1+ R2
R2
Where VFB is the feedback voltage and VOUT is
the output voltage.
A typical value for R2 can be as high as 100kΩ,
but a typical value is 10kΩ. Using that value, R1
is determined by:
R1 = 8.18 × (VOUT − 1.23)(kΩ)
For example, for a 3.3V output voltage, R2 is
10kΩ, and R1 is 17kΩ.
Inductor
The inductor is required to supply constant
current to the output load while being driven by
the switched input voltage. A larger value
inductor will result in less ripple current that will
result in lower output ripple voltage. However,
the larger value inductor will have a larger
physical size, higher series resistance, and/or
lower saturation current. A good rule for
determining the inductance to use is to allow
the peak-to-peak ripple current in the inductor
to be approximately 30% of the maximum
switch current limit. Also, make sure that the
MP1570 Rev. 1.5
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5
1/31/2006
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