SP6121CN-L Datasheet, PDF (9/42 Pages) Sipex Corporation – Low Voltage, Synchronous Step Down PWM Controller

while the inductor current ramps up or down to
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 SP6121 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
contributor to output voltage ripple. The maxi-
mum allowable ESR required to maintain a
specified output voltage ripple can be calculated
by:
R_ESR
≤
∆V_OUT
I _PP
where;
∆V_OUT = peak to peak output voltage ripple
I_PP = 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 =}
I_PP (1 – D)
C_OUTF_S
2 _{+ (IPPRESR)2}
where;
D = duty cycle equal to V_OUT/V_IN
C_OUT = output capacitance value
Recommended capacitors that can be used ef-
fectively in SP6121 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 SP6121
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.
APPLICATIONS INFORMATION
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 V_OUT/ V_IN. 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:
I_CIN(rms) = I_{OUT(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:
P = I R ²
C IN
CIN ( rms) ESR( CIN)
This can become a significant part of power
losses in a converter and reduce 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:
∆V_IN
=
I_OUT(MAX)R_ESR(CIN)
+
I_OUT(MAX)V_OUT(V_IN
F_SC_INV_IN2
-
V_OUT)
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
Date: 11/29/04
SP6121 Low Voltage, Synchronous Step Down PWM Controller
9
© Copyright 2004 Sipex Corporation