PE99151_14 Datasheet, PDF (10/15 Pages) Peregrine Semiconductor – Radiation Hardened UltraCMOS Monolithic Point-of-Load Synchronous Buck Regulator with Integrated Switches

PE99151 DIE
Product Specification
DESIGN GUIDE
Setting the Output Voltage
The PE99151 can be configured to output a DC voltage
from +1.0V to +3.6V. The user can set the output voltage by
selecting the external feedback resistors R_fb1 and R_fb2. The
feedback resistors divide down the output voltage to be
compared to a +1.000V reference voltage. The error
amplifier uses this comparison to determine the amount of
current to send to the load.
To set the output voltage, a resistor divider must selected
that will produce at +1.000V DC voltage at the EAINM pin
when V_OUT has reached the target output voltage.
Figure 6. Output Voltage Selection
Vout ^Rfb1
Rfb2
_ EAINM
+ ^EAINP
Error Amp
1.000V
VREF
V_OUT = (R_fb1+R_fb2)/R_fb2 = 1 + R_fb1/R_fb2, and
R_fb1 = R_fb2* (V_OUT - 1), +1V <V_OUT ≤+3.6V
The PE99151 reference design uses a value of 10 kΩ for
R_fb2.
Example:
Desired V_OUT = +2.5V
R_fb2 = 10 kΩ
R_fb1 = 10 kΩ * (+2.5V - 1) = 15 kΩ
* For a desired output voltage of 1V, Rfb1 can be replaced with a 0 Ω resistor and
Rfb2 not installed. This is equivalent to directly connecting VOUT to EAINM.
Output Inductor Selection
The output Inductor serves as the main energy storage
element in a switching regulator. It is perhaps the most
critical component influencing the performance of the buck
regulator. It impacts many aspects of the power supply
system performance, including power supply bandwidth,
output voltage ripple and ripple spectrum, and switching,
conduction, and core losses. Additionally, specific aspects
of the buck regulator itself place requirements on the range
of allowable Inductor values. These aspects include the
internal current detector sensitivity, the slope
compensation ramp dynamic range, and the current
limitations of the part. The selection of the Inductor is
also a function of the specifics of the application
including input voltage, output voltage, load current
range, switching frequency, PCB area, efficiency targets,
power supply bandwidth, and ripple requirements, to
name a few.
Many performance requirements and other component
selections place restrictions on the Inductor selection.
However, since the Inductor selection plays a central
role in the performance of the power supply, its selection
needs to be made early in the design process.
Therefore, as a starting point, the Inductor needs to be
initially selected based on a few rough calculations and
selection can be refined iteratively as more system
requirements are introduced.
The voltage across the Inductor is V_L = L x ∆IL/∆t, where
∆IL is defined to be the Inductor peak-to-peak current
ripple. The ripple current is the change in the Inductor
current during each switching cycle. For the PE99151, the
lower limit of ∆IL is set by the current threshold comparator
sensitivity, while the upper limit of ∆IL is set by the current
mode compensation dynamic range.
Given the output voltage, switching frequency, input
voltage and the minimum ∆IL required by the part, the
Inductance can be calculated as:
L = V_L x ∆t/∆IL
L = V_OUT/(F_SW x ∆IL) x [1 – D], where
Duty cycle = D = V_OUT/V_IN
Switching frequency = F_SW
Duration of Inductor voltage = ∆t = D/F_SW
As the output switches pull the OUT pin alternately to V_IN
and to GND, the inductor peak to peak current ripple
(triangular current waveform magnitude) is expressed as:
∆IL = V_OUT/(L x F_SW) * (1 – D)
Example:
V_IN = +5.0V
V_OUT = +2.5V
F_SW = 1 MHz
∆IL = 0.5A
L = V_OUT/(F_SW x ∆IL) x [1 – D]
L = (+2.5/(1 MHz x 0.5) * [(1 – (+2.5/+5.0))] = 2.5 µH
The Inductor self resonant frequency (SRF) should be
selected to be at least 10x higher than the switching
frequency F_SW. Meeting this requirement will ensure
stability, reduce output ripple and improve efficiency.
©2012-2014 Peregrine Semiconductor Corp. All rights reserved.
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Document No. DOC-50370-3 │ UltraCMOS^® Power Management Solutions