SI9161 Datasheet, PDF (10/12 Pages) Vishay Siliconix – Optimized-Efficiency Controller for RF Power Amplifier Boost Converter

Si9161
Vishay Siliconix
Stability Components
A voltage mode boost converter is normally stabilized with
simple lag compensation due to the additional 90° phase lag
introduced by the additional right hand plane zero, as well as
having a duty factor dependent resonant frequency for the
output filter. The stability components shown in Figure 1 have
been chosen to ensure stability under all battery conditions
while maintaining maximum transient response. To do this we
have used simple lag compensation (type 1 amplifier
configuration). Figure 4 shows the bode plot for the above
circuit, maintaining > 50° phase margin over the entire battery
voltage range.
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.
L
=
-^V----^I2-^N-----^â‹---⁽--^V-----^O----^U---^T----^–-----^V----^I--^N---⁾----^â‹---^η--
2 ⋅ V_{O2 UT} ⋅ I_OUT ⋅ f
η = efficiency
FIGURE 4. Stability, with 1-cell Li battery input, 5 V @
600-mA output.
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-Ω 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.
C
=
-^I--^O---^U----^T----^â‹---⁽---^V----^O----^U---^T-----^–----^V----^I--^N---⁾-
V_OUT ⋅ ∆V_RIPPLE ⋅ f
I_OUT
V_OUT
V_IN
∆V_RIPPLE
f
= output dc load current
= output voltage
= input voltage
= desired output ripple voltage
= switching frequency
FIGURE 5. Continous and Discontinous Inductance Curve
Designed with small surface mount inductors and capacitors,
the Si9161 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.
RESULTS SECTION
The following section shows the actual results obtained with
the circuit diagram shown in Figure 1.
Efficiency
The graph below 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.
S-60752—Rev. B, 05-Apr-99
10
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