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RT6203B Datasheet(PDF) 18 Page - Richtek Technology Corporation

Part No. RT6203B
Description  5A, 18V, 700kHz ACOTTM Synchronous Step-Down Converter with VID Control
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Maker  RICHTEK [Richtek Technology Corporation]
Homepage  http://www.richtek.com
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 18 page
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RT6203B
18
DS6203B-01 July 2017
www.richtek.com
©
Copyright 2017 Richtek Technology Corporation. All rights reserved.
is a registered trademark of Richtek Technology Corporation.
reduce voltage ringing and overshoot.
Choose capacitors rated at higher temperatures than
required. Several ceramic capacitors may be paralleled to
meet the RMS current, size, and height requirements of
the application.
Output Capacitor Selection
The RT6203B 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.
Output Ripple Voltage
Output ripple voltage at the switching frequency is caused
by the inductor current ripple and its effect on the output
capacitor's ESR and stored charge. These two ripple
components are called ESR ripple and capacitive ripple.
Since ceramic capacitors have extremely low ESR and
relatively little capacitance, both components are similar
in amplitude and both should be considered if ripple is
critical.
RIPPLE
RIPPLE(ESR)
RIPPLE(C)
RIPPLE(ESR)
L
ESR
L
RIPPLE(C)
OUT
SW
V
= V
V
V
= I
R
I
V
=
8C
f


Output Transient Undershoot and Overshoot
In addition to output ripple voltage at the switching
frequency, the output capacitor and its ESR also affect
the voltage sag (undershoot) and soar (overshoot) when
the load steps up and down abruptly. The ACOTTM transient
response is very quick and output transients are usually
small. However, the combination of small ceramic output
capacitors (with little capacitance), low output voltages
(with little stored charge in the output capacitors), and
low duty cycle applications (which require high inductance
to get reasonable ripple currents with high input voltages)
increases the size of voltage variations in response to
very quick load changes. Typically, load changes occur
slowly with respect to the IC's switching frequency.
But some modern digital loads can exhibit nearly
instantaneous load changes and the following section
shows how to calculate the worst-case voltage swings in
response to very fast load steps.
The output voltage transient undershoot and overshoot each
have two components : the voltage steps caused by the
output capacitor's ESR, and the voltage sag and soar due
to the finite output capacitance and the inductor current
slew rate. Use the following formulas to check if the ESR
is low enough (typically not a problem with ceramic
capacitors) and the output capacitance is large enough to
prevent excessive sag and soar on very fast load step
edges, with the chosen inductor value.
The amplitude of the ESR step up or down is a function of
the load step and the ESR of the output capacitor :
ESR_STEP
OUT
ESR
V
= I
R

The amplitude of the capacitive sag is a function of the
load step, the output capacitor value, the inductor value,
the input-to-output voltage differential, and the maximum
duty cycle. The maximum duty cycle during a fast transient
is a function of the on-time and the minimum off-time since
the ACOTTM control scheme will ramp the current using
on-times spaced apart with minimum off-times, which is
as fast as allowed. Calculate the approximate on-time
(neglecting parasitic) and maximum duty cycle for a given
input and output voltage as :
OUT
ON
ON
MAX
IN
SW
ON
OFF(MIN)
Vt
t
=
and D
=
V
f
t
+ t
The actual on-time will be slightly longer as the IC
compensates for voltage drops in the circuit, but we can
neglect both of these since the on-time increase
compensates for the voltage losses. Calculate the output
voltage sag as :

2
OUT
SAG
OUT
IN(MIN)
MAX
OUT
L( I
)
V
=
2C
V
D
V


The amplitude of the capacitive soar is a function of the
load step, the output capacitor value, the inductor value
and the output voltage :
2
OUT
SOAR
OUT
OUT
L( I
)
V
=
2C
V






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