XRP6657 Datasheet, PDF (8/11 Pages) Exar Corporation – 1.5A 1.3MHz Synchronous Step Down Converter

XRP6657
1.5A 1.3MHz Synchronous Step Down Converter
THEORY OF OPERATION
The typical application circuit is shown below.
OUTPUT VOLTAGE
The adjustable output voltage is determined
by:
Eq. 4:
0.6 1
Fig. 22: Typical Application Circuit
INDUCTOR SELECTION
Inductor ripple current and core saturation are
two factors considered to select the inductor
value.
Eq. 1: ∆
1
Equation 1 shows the inductor ripple current
as a function of the frequency, inductance, V_IN
and V_OUT. It is recommended to set the ripple
current to 40% of the maximum load current.
A low ESR inductor is preferred.
SHORT CIRCUIT BEHAVIOR
The XRP6657 has an over current and over
temperature protection. The over current
applies cycle by cycle and limits the P-driver
FET current to maintain the inductor current
within safe limits. The over temperature
protection circuitry turns off the driver FETs
when the junction temperature is too high.
Normal Operations are restored when
temperature drops below the safety threshold.
In the following example, the XRP6657 is used
to convert a 5V input to a 1.2V output.
Shorting VOUT to ground triggers both the
over current and over temperature protection
circuits. The waveform is shown below.
C_IN AND C_OUT SELECTION
A low ESR input capacitor can prevent large
voltage transients at V_IN. The RMS current
rating of the input capacitor is required to be
larger than I_RMS calculated by:
Eq. 2:
The ESR rating of the capacitor is an important
parameter to select C_OUT. The output ripple
V_OUT is determined by:
Eq. 3: ∆
∆
Higher values, lower cost ceramic capacitors
are now available in smaller sizes. These
capacitors have high ripple currents, high
voltage ratings and low ESR that makes them
ideal for switching regulator applications. As
C_OUT does not affect the internal control loop
stability, its value can be optimized to balance
very low output ripple and circuit size. It is
recommended to use an X5R or X7R rated
capacitors which have the best temperature
and voltage characteristics of all the ceramics
for a given value and size.
Fig. 23: Short Circuit Response
THERMAL CONSIDERATIONS
Although the XRP6657 has an on board over
temperature circuitry, the total power
dissipation it can support is based on the
package thermal capabilities. The formula to
ensure safe operation is given in note 1 under
the operating ratings section.
To avoid exceeding the maximum junction
temperature, thermal analysis is strongly
suggested.
© 2009 Exar Corporation
8/11
Rev. 1.0.0