DS6257AB Datasheet, PDF (10/15 Pages) –

RT6257A/B
Application Information
Inductor Selection
The consideration of inductor selection includes
inductance, RMS current rating and, saturation current
rating. The inductance selection is generally flexible and
is optimized for the low cost, low physical size, and high
system performance.
Choosing lower inductance to reduce physical size and
cost, and it is useful to improve the transient response.
However, it causes the higher inductor peak current and
output ripple voltage to decrease system efficiency.
Conversely, higher inductance increase system efficiency,
but the physical size of inductor will become larger and
transient response will be slow because more transient
time is required to change current (up or down) by inductor.
A good compromise between size, efficiency, and transient
response is to set a inductor ripple current (ΔI_L) about
20% to 50% of the desired full output load current.
Calculate the approximate inductance by the input voltage,
output voltage, switching frequency (f_SW), maximum rated
output current (I_OUT(MAX)) and inductor ripple current (ΔI_L).
L=
V_OUT   V_IN  V_OUT 
V_IN  f_SW  I_L
Once the inductance is chosen, the inductor ripple current
(ΔI_L) and peak inductor current can be calculated.
I_L
=
V_OUT 
V_IN
 VIN  VOUT
 f_SW L

I_L(PEAK)
=
I_OUT(MAX)

1
2
I_L
I_L(VALLY)
=
I_OUT(MAX)

1
2
I_L
The typical operating circuit design for the RT6257A/B,
the output voltage is 5V, maximum rated output current
is 6A, input voltage is 12V, and inductor ripple current is
1.8A which is 30% of the maximum rated output current,
the calculated inductance value is :
L
=
5  12  5
12 500 10³ 1.8
= 3.24μH
The inductor ripple current set at 1.8A and so we select
3.3μH inductance. The actual inductor ripple current and
required peak current is shown as below :
I_L
=
12

5  12
500 10³


5
3.3
10^-6
= 1.77A
I_L(PEAK)
=
I_OUT(MAX)

1
2
I_L
=6+
1.77
2
= 6.885A
Inductor saturation current should be chosen over IC's
valley current limit.
Input Capacitor Selection
The effective input capacitance is a function of the input
voltage (V_IN), output voltage (V_OUT), rated output current
(I_OUT), switching frequency (f_SW), and input ripple voltage
of the regulator (ΔV_INP) :
C_IN(MIN)
=
I_OUT

V_OUT
V_IN
 ^{}1
V_OUT
V_IN
f_SW  V_INP

Ceramic capacitors are most often used because of their
low cost, small size, high RMS current ratings, and robust
surge current capabilities. It should pay attention that value
of capacitors change as temperature, bias voltage, and
operating frequency change. For example the capacitance
value of a capacitor decreases as the dc bias across the
capacitor increases. Several ceramic capacitors may be
paralleled to meet the RMS current, size, and height
requirements of the application. Considering the DC bias
effects for the input capacitor, the typical operating circuit
used two 10μF low ESR ceramic capacitors on the VIN
pin and an additional 0.1μF is recommended to place as
close as possible to the IC input side for high frequency
filtering.
Output Capacitor Selection
The RT6257A/B is optimized for output terminal with
ceramic capacitors application and best performance will
be obtained using them. The total output capacitance value
is usually determined by the desired output ripple voltage
level and transient response requirements for sag which
is undershoot on positive load steps and soar which is
overshoot on negative load steps.
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DS6257A/B-00 September 2016