SI9160 Datasheet, PDF (10/11 Pages) Vishay Siliconix – Controller for RF Power Amplifier Boost Converter

Si9160
Vishay Siliconix
Energy Storage Components
The input and output ripple voltage is determined by the
switching frequency, and the inductor and capacitor values.
The higher the frequency, inductance, or capacitance values,
the lower the ripple. The efficiency of the converter is also
improved with higher inductance by reducing the conduction
loss in the switch, synchronous rectifier, and the inductor itself.
In the past, Tantalum was the preferred material for the input
and output capacitors. Now, with 2-MHz switching
frequencies, Tantalum capacitors are being replaced with
smaller surface mount ceramic capacitors. Ceramic
capacitors have almost no equivalent series resistance (ESR).
Tantalum capacitors have at least 0.1-W ESR. By reducing
ESR, converter efficiency is improved while decreasing the
input and output ripple voltage. With ceramic capacitors,
output ripple voltage is a function of capacitance only. The
equation for determining output capacitance is stated below.
I_OUT @ (V_OUT – V_IN)
C + _{VOUT @ DVRIPPLE @ f}
I_OUT = output dc load current
V_OUT = output voltage
V_IN = input voltage
DV_RIPPLE = desired output ripple voltage
f = switching frequency
The inductance value for the converter is a function of the
desired ripple voltage and efficiency as stated below. In order
to keep the ripple small and improve efficiency, the inductance
needs to be large enough to maintain continuous current
mode. Continuous current mode has lower RMS current
compared to discontinuous current mode since the peak
current is lower. This lowers the conduction loss and improves
efficiency. The equation that shows the critical inductance
which separates continuous and discontinuous current mode
at any given output current is stated below. This equation is
also plotted in Figure 5 as a function of input voltage.
Designed with small surface mount inductors and capacitors,
the Si9160 solution can fit easily within a small space such as
a battery pack. Another distinct advantage of a smaller
converter size is that it reduces the noise generating area by
reducing the high current path; therefore radiated and
conducted noise is less likely to couple into sensitive circuits.
ǒ Ǔ _L
+
V_IN ²
2@
@ V_OUT – V_IN
V_OUT ² @ I_OUT
@h
@f
h = efficiency
0.7
Continuous Mode
0.6
Discontinuous Mode
0.5
h = 0.9
0.4
V_OUT = 5 V
I_OUT = 0.5 A
f = 1 MHz
0.3
2.5
3.0
3.5
4.0
4.5
V_IN (V)
FIGURE 5. Continuous and Discontinuous Inductance Curve
RESULTS SECTION
The following section shows the actual results obtained with
the circuit diagram shown in Figure 1.
Efficiency
The Figure 6 shows the efficiency of the above design at
various constant switching frequencies. The frequencies were
generated using a 3-V square wave of the desired frequency
to the sync input to the circuit. The input voltage to the circuit
is 3.6-V dc.
Output Noise
The noise generated by a dc-dc converter is always an issue
within the mobile phone. The Si9160 offers two benefits.
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10
The noise spectrum is a constant, i.e. no random noise or
random harmonic generation.
The switching fundamental can be synchronized to a
known frequency, e.g. 812.5 kHz which is ¹/₁₆-th of the
GSM/DCS system clock, 1.23 MHz which is the channel
spacing frequency for CDMA, etc.
Figures 7 through 9 show the output noise and output
spectrum analysis.
Output Noise Spectrum
Note there is no random noise, only switching frequency
harmonics. This is very good news for the RF stages, where
an unknown, or random noise spectrum will cause problems.
Document Number: 70029
S-40700—Rev. H, 19-Apr-04