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SP6123A Datasheet(PDF) 10 Page - Sipex Corporation

Part # SP6123A
Description  Low Voltage, Synchronous Step-Down PWM Controller Ideal for 2A to 10A, Small Footprint, DC-DC Power Converters
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Manufacturer  SIPEX [Sipex Corporation]
Direct Link  http://www.sipex.com
Logo SIPEX - Sipex Corporation

SP6123A Datasheet(HTML) 10 Page - Sipex Corporation

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Date: 5/25/04
SP6123 Low Voltage, Synchronous Step Down PWM Controller
© Copyright 2004 Sipex Corporation
the value corresponding to the new load current.
Additionally, the ESR in the output capacitor
causes a step in the output voltage equal to the
ESR value multiplied by the change in load
current. Because of the fast transient response
provided by the SP6123 when exposed to output
load transient, the output capacitor is typically
chosen for ESR , not for capacitance value.
The output capacitor’s ESR, combined with the
inductor ripple current, is typically the main con-
tributor to output voltage ripple. The maximum
allowable ESR required to maintain a specified
output voltage ripple can be calculated by:
PP
OUT
ESR
I
V
R
where:
∆VOUT = peak to peak output voltage ripple
IPP = peak to peak inductor ripple current
The total output ripple is a combination of the
ESR and the output capacitance value and can
be calculated as follows:
∆VOUT =
(IPP (1–D))2+(IPPRESR)2
COUTFS
where:
D = duty cycle equal to VOUT/VIN
COUT = output capacitance value
Recommended capacitors that can be used ef-
fectively in SP6123 applications are: low-ESR
aluminum electrolytic capacitors, OS-CON ca-
pacitors that provide a very high performance/
size ratio for electrolytic capacitors and low-
ESR tantalum capacitors. AVX TPS series and
Kemet T510 surface mount capacitors are popu-
lar tantalum capacitors that work well in SP6123
applications. POSCAP from Sanyo is a solid
electrolytic chip capacitor that has low ESR and
high capacitance. For the same ESR value,
POSCAP has lower profile compared with tan-
talum capacitor.
Panasonic offers the SP series of specialty poly-
mer aluminum electrolytic surface mount ca-
pacitors. These capacitors have a lower ESR
than tantalum capacitors, reducing the total num-
ber of capacitance required for a given transient
response.
Input Capacitor Selection
The input capacitor should be selected for ripple
current rating, capacitance and voltage rating.
The input capacitor must meet the ripple current
requirement imposed by the switching current.
In continuous conduction mode, the source cur-
rent of the high-side MOSFET is approximately
a square wave of duty cycle VOUT/ VIN. Most of
this current is supplied by the input bypass
capacitors. The RMS value of input capacitor
current is determined at the maximum output
current and under the assumption that the peak
to peak inductor ripple current is low, it is given
by:
ICIN(rms) = IOUT(max) √D(1 - D)
The worse case occurs when the duty cycle, D,
is 50% and gives an RMS current value equal to
IOUT/2. Select input capacitors with adequate
ripple current rating to ensure reliable opera-
tion.
The power dissipated in the input capacitor is:
)
(
2
)
(
CIN
ESR
rms
CIN
CIN
R
I
P
=
This can become a significant part of power
losses in a converter and hurt the overall energy
transfer efficiency.
The input voltage ripple primarily depends on
the input capacitor ESR and capacitance. Ignor-
ing the inductor ripple current, the input voltage
ripple can be determined by:
2
)
(
)
(
(max)
)
(
IN
IN
S
OUT
IN
OUT
MAX
OUT
CIN
ESR
out
IN
V
C
F
V
V
V
I
R
I
V
+
=
The capacitor type suitable for the output ca-
pacitors can also be used for the input capaci-
tors. However, exercise extra caution when tan-
talum capacitors are considered. Tantalum ca-
pacitors are known for catastrophic failure when
exposed to surge current, and input capacitors
are prone to such surge current when power
supplies are connected ‘live’ to low impedance
APPLICATIONS INFORMATION


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