XRP6657
1.5A 1.3MHz Synchronous Step Down Converter
THEORY OF OPERATION
OUTPUT VOLTAGE
The typical application circuit is shown below.
The adjustable output voltage is determined
by:
Eq. 4: ꢆꢇꢈꢉ ꢂ 0.6ꢆ ꢅ ꢊ1 ꢠ ꢓꢓꢣꢒ
ꢤ
S
HORT 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.
Fig. 22: Typical Application Circuit
I
NDUCTOR SELECTION
Inductor ripple current and core saturation are
two factors considered to select the inductor
value.
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.
ꢃ
Eq. 1: ∆ꢀꢁ ꢂ ꢄꢅꢁ ꢆꢇꢈꢉ ꢊ1 ꢋ ꢌꢍꢎꢏꢒ
ꢌ
ꢐꢑ
Equation 1 shows the inductor ripple current
as a function of the frequency, inductance, VIN
and VOUT. It is recommended to set the ripple
current to 40% of the maximum load current.
A low ESR inductor is preferred.
CIN AND COUT SELECTION
A low ESR input capacitor can prevent large
voltage transients at VIN. The RMS current
rating of the input capacitor is required to be
larger than IRMS calculated by:
ꢚ
ꢜ
Eq. 2: ꢀꢓꢔꢕ ꢖ ꢀꢇꢔꢗꢘ ꢅ ꢙꢌ
ꢅ
ꢌ
ꢛꢌ
ꢍꢎꢏ
ꢍꢎꢏ
ꢐꢑ
ꢌ
ꢍꢎꢏ
The ESR rating of the capacitor is an important
parameter to select COUT. The output ripple
VOUT is determined by:
ꢃ
Fig. 23: Short Circuit Response
Eq. 3: ∆ꢆꢇꢈꢉ ꢖ ∆ꢀꢁ ꢊꢝꢞꢟ ꢠ ꢡꢅꢄꢅꢢ
ꢒ
ꢍꢎꢏ
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
COUT 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.
T
HERMAL 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
EXAR [ EXAR CORPORATION ]
