XRP6658 Datasheet, PDF (8/11 Pages) Exar Corporation – 1A 1.5MHz PFM/PWM Synchronous Step-Down Converter

XRP6658
1A 1.5MHz PFM/PWM Synchronous Step-Down Converter
THEORY OF OPERATION
The typical application circuit of adjustable
version is shown in figure 22.
Fig. 22: Typical Application
INDUCTOR SELECTION
Inductor ripple current and saturation current
rating are two factors to be considered when
selecting the inductor value. A low DCR
inductor is preferred.
The inductor value L can be calculated from
the following equation:
x¹x ¹
∆
C_IN AND C_OUT SELECTION
A low ESR input capacitor can minimize the
input voltage ripple. Voltage rating of the
capacitor should be at least 50% higher than
the input voltage. The RMS current of the
input capacitor is required to be larger than
the I_RMS calculated by:
≅
-
The ESR value is an important parameter to
consider when selecting an output capacitor
C_OUT. The output ripple V_OUT is determined by:
1
∆ ≅ ∆I_L ESR _{8 f}
The output capacitor’s value can be optimized
for very low output voltage ripple and small
circuit size. Voltage rating of the capacitor
should be at least 50% higher than the output
voltage. Higher values, lower cost ceramic
capacitors are now available in smaller sizes.
These ceramic capacitors have high ripple
currents, high voltage ratings and low ESR
that make them ideal for switching regulator
applications.
It is recommended to use X5R or X7R ceramic
capacitors as they have the best temperature
and voltage characteristics.
OUTPUT VOLTAGE SELECTION
The output voltage is adjustable via the
external resistor network R1 and R2 as per the
following formula:
R2
1 _R1
where, V_REF is the reference voltage at 0.6V.
The feedback resistors must be chosen such
that power dissipation of the network is
minimal. R1 must be selected within the range
of 80kΩ≤ R1 ≤120kΩ. R2 is selected based on
the above equation.
THERMAL CONSIDERATIONS
Although thermal shutdown is built-in in
XRP6658 to protect the device from thermal
damage, the total power dissipation that
XRP6658 can sustain is based on the package
thermal capability. The formula to ensure safe
operation is shown in Note 1. To avoid
XRP6658 from exceeding the maximum
junction temperature, some thermal analysis
is required.
GUIDELINES FOR PCB LAYOUT
To ensure proper operation of the XRP6658,
please note the following PCB layout
guidelines:
1. The GND, SW and VIN traces should be
kept short, direct and wide.
2. VFB pin must be connected directly to the
feedback resistors. Resistive divider R1/R2
must be connected in parallel to the output
capacitor C_OUT.
3. The input capacitor C_IN must be as close as
possible to pin VIN.
4. Keep SW node away from the sensitive VFB
node since SW signal experiences high
frequency voltage swings.
© 2011 Exar Corporation
8/11
Rev. 1.5.0