DS6203B Datasheet, PDF (17/22 Pages) Richtek Technology Corporation – 5A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter with VID Control

RT6203B
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 RT6203B, the
output voltage is 1.2V, maximum rated output current is
5A, input voltage is 12V, and inductor ripple current is 1A
which is 20% of the maximum rated output current, the
calculated inductance value is :
L
=
1.2 12 1.2
12 700 10³ 1
=
1.53μH
The inductor ripple current can be set larger than 1A and
so we select 1.5μH inductance. The actual inductor ripple
current and required peak current is shown as below :
I_L
=
12

1.2 12
700 10³
 1.2 
1.5 
10^-6
= 1.02A
Copyright ©2016 Richtek Technology Corporation. All rights reserved.
DS6203B-00 September 2016
I_L(PEAK)
=
I_OUT(MAX)

1
2
I_L
=5+
1
2
= 5.5A
Inductor saturation current should be chosen over IC’s
current limit. Set valley current limit of the RT6203B is 7A
by I²C. When touching current limit of the RT6203B, the
peak inductor current is :
I_L(PEAK) = I_L(VALLEY) + I_L = 7 + 1 = 8A
It will be safe to choose inductor saturation current larger
than 8.1A.
Input Capacitor Selection
The input filter capacitors are needed to smooth out the
RMS input ripple current drawn from the input power source
and ripple voltage seen at the input of the converter. The
voltage rating of the input filter capacitors must be greater
than the maximum input voltage. It's also important to
consider the ripple current capabilities of capacitors.
The RMS input ripple current (I_RMS) is a function of the
input voltage (V_IN), output voltage (V_OUT), and rated output
current (I_OUT) :
I_RMS
=
I_OUT 
V_OUT
V_IN

V_IN
V_OUT
1
The maximum RMS input ripple current occurs at
maximum output load and it needs to be concerned about
the ripple current capabilities of capacitors at maximum
output load.
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.
However, take care when these capacitors are used at
the input of circuits supplied by a wall adapter or other
supply connected through long and thin wires. Current
surges through the inductive wires can induce ringing at
the IC's power input which could potentially cause large,
damaging voltage spikes at VIN pin. If this phenomenon
is observed, some bulk input capacitance may be
required. Ceramic capacitors can be placed in parallel with
other types such as tantalum, electrolytic, or polymer to
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