X RP7 6 6 5
3 A 1 8 V S y n c h r o n o u s S t e p - D o w n Co n v e r t e r
If tantalum or electrolytic capacitors are used
V
OUT
is essentially a function of ESR:
Circuit configuration is
shown in figures 16 and
17. The external bootstrap diode is also
recommended where dut
y cycle (V
OUT
/V
IN
) is
larger than 65%.
INPUT CAPACITOR
C
IN
1N4148
V
I N
= 5V
IN
BS
10nF
SW
Select the input capacitor for voltage rating,
RMS current rating and capacitance. The
voltage rating
should be at least 50% higher
than the regulator
maximum input voltage.
XRP7 6 6 5
Where:
I
OUT
D is duty cycle (D=V
Calculate the C
OUT
/V
IN
)
3A)
Fig. 16 : Optional external bootstrap diode
where input voltage is fixed at 5V
IN
capacitance from:
1N4148
BS
Where:
is the permissible input voltage ripple,
nominally set at 1% of V
IN
IN
XRP7 6 6 5
SW
10nF
V
OUT
= 5V
or 3.3V
C
OUT
O
PTIONAL
S
CHOTTKY DIODE
An optional Schottky diode may be paralleled
between the GND pin and S
W pin to improve
the regulator efficiency. Examples are shown
in Table 3.
Fig. 17 : Optional external
bootstrap diode
where output voltage is 5V or 3.3V
LOOP COMPENSATION
Part Number
B130
SK13
MBRS130
Voltage/Current
Rating
30V/1A
30V/1A
30V/1A
Vend or
Diodes, Inc.
Diodes, Inc.
International
Rectifier
Table 3. Optional Schottky diode
EXTERNAL BOOTSTRAP DIODE
A low - cost diode , such as 1N4148 , is
recommended for higher efficiency when
the
input voltage is 5V or the output is 5V or 3.3V.
© 2013 Exar Corporation
9/ 12
XRP7665 utilizes current - mode control . This
allows using a minimum of external
components to compensate the regulator. In
general only two components are needed: RC
and CC. Proper compens ation of the regulator
(determining RC and CC) results in optimum
transient response.
In terms of power supply
control theory, t he goals of compensation are
to choose RC and CC such that the regulator
loop gain has a crossover frequency fc
between 15kHz an d 34kHz. The corresponding
phase - margin should be between 45
degrees
and 65 degrees. An important characteristic of
current - mode buck regulator is its dominant
pole. The frequency of the
dominant pole is
given by:
Rev. 2.0. 1
EXAR [ EXAR CORPORATION ]
