XRP6657_12 Datasheet, PDF (8/11 Pages) Exar Corporation – 1.5A 1.3MHz Synchronous Step Down Converter

XRP6 6 5 7
1 .5 A 1 .3 M H z Sy n ch r o n o u s St e p D o w n Co n v e r t e r
THEORY OF OPERATION
The typical application circuit is shown below.
Fig. 22 : Typical Application Circuit
I NDUCTOR SELECTION
Inductor ripple current and core satu
ration are
two factors considered to select the inductor
value.
Eq. 1: οܫ_à¯àµŒ_{௙ൈଵà¯}ܸ_{ை௎்}ቀͳെ^{௏௏ೀ಺ೆಿà³}ቁ
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 between 30% to 40% of the maximum
load current. A low ESR inductor is preferred.
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: ܫ_{ோெௌ}؆ܫ_{ைெ஺௑}ൈ^{ඥ௏ೀೆ೅ൈ௏ሺ௏}_{಺಺ಿಿ}^{ି௏ೀೆ೅ሻ}
The ESR rating of the capacitor is an important
parameter to select C _OUT. The output ripple
V_OUT is determined by:
Eq. 3: οܸ_{ை௎்}؆οܫ_à¯á‰€Ü´ÜµÜ§àµ…_{଼ൈ௙ൈଵ஼ೀೆà³}ቁ
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.
OUTPUT V OLT AGE
The adjustable output voltage is determined
by:
Eq. 4: ܸ_{ை௎்}ൌͲǤ͸ܸൈቀͳ൅^{ோோభమ}ቁ
SHORT CIRCUIT 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 mainta in 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 ex ample, 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. 23 : Short Circuit Response
THERM AL 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.
© 2012 Exar Corporation
8/ 11
Rev. 1.4.0