SP6120 Datasheet, PDF (15/22 Pages) Sipex Corporation – Low Voltage, AnyFETTM, Synchronous ,Buck Controller Ideal for 2A to 10A, High Performance, DC-DC Power Converters

APPLICATIONS INFORMATION: Continued
SP6120 adjusts the inductor current to the new
value. Therefore the capacitance must be large
enough so that the output voltage is held up
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
and inherent 100% and 0% duty cycle capabil-
ity provided by the SP6120 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:
where:
R_ESR
≤
∆V_OUT
I _PP
∆V_OUT = peak to peak output voltage ripple
I_PP = peak to peak inductor ripple current
Kemet T510 surface mount capacitors are popu-
lar tantalum capacitors that work well in SP6120
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.
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
I_OUT /2. Select input capacitors with adequate
ripple current rating to ensure reliable opera-
tion.
The total output ripple is a combination of the
ESR and the output capacitance value and can
be calculated as follows:
The power dissipated in the input capacitor is:
P = I R ²
C IN
CIN ( rms) ESR( CIN)
( ) ^{∆VOUT =}
I_PP (1 – D)
C_OUTF_S
2
+ (I_PPR_ESR)²
where:
F_S = switching frequency
D = duty cycle
C_OUT = output capacitance value
Recommended capacitors that can be used ef-
fectively in SP6120 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
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:
∆ V_IN
= I _out(max)
R_{E SR(CIN )}
+
I _{OUT (MAX} V_{) OUT} (V_IN
F_SC _INV_IN ²
−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 tanta-
Date: 1/21/05
SP6120 Low Voltage, AnyFET^TM, Synchronous, Buck Controller
15
© Copyright 2005 Sipex Corporation