X RP 6 6 5 7
1.5A 1.3MHz Synchronous Step Down Converter
THEORY OF OPERATION
The typical application circuit is shown below.
O
UTPUT
V
OLTAGE
The adjustable output voltage is determined
by:
Eq. 4:
0.6
1
S
HORT
C
IRCUIT
B
EHAVIOR
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.
Fig. 22: Typical Application Circuit
I
NDUCTOR
S
ELECTION
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.
C
IN
AND
C
OUT
S
ELECTION
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:
∆
∆
Fig. 23: Short Circuit Response
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.
© 2009 Exar Corporation
8/11
T
HERMAL
C
ONSIDERATIONS
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.
Rev. 1.0.0
EXAR [ EXAR CORPORATION ]
