AN732 Datasheet, PDF (1/7 Pages) Vishay Siliconix – Designing A High-Voltage Non-Isolated Buck-Boost Converter with the Si9121DY

AN732
Vishay Siliconix
Designing A High-Voltage Non-Isolated
Buck-Boost Converter with the Si9121DY
Nitin Kalje
INTRODUCTION
The Si9121DY is a non-isolated buck-boost converter IC,
operating from a wide input voltage range of –10 to –60 V with
minimal external components. This polarity inverter converts
–48 V to +5 V or +3.3 V, making it suitable for applications
including digital phones and ISDN power supplies. A
non-isolated buck-boost design with the Si9121 eliminates the
need for an expensive transformer, while its integrated low
on-resistance MOSFET driver, floating feedback error
amplifier, fixed-frequency oscillator, output voltage sensing
resistor divider, and depletion mode MOSFET for start up/V_CC
regulation reduce the external component count to less
than 10. Current-mode control technology achieves good line
transient response, making the Si9121DY suitable for the high
source impedance environment. Other features include the
under-voltage lock out, programmable soft start, pulse by
pulse current limit, hiccup mode with negligible power delivery
and dissipation during continuous short circuit conditions,
automatic recovery from hiccup after fault removal, and over
temperature shutdown. The Si9121 is available in a narrow
body SO-8 package and allows power dissipation of up to
1.25 W.
DEVICE OPERATION
The Si9121DY uses a current-mode flyback topology to
convert the –48-V input to a +5-V or +3.3-V output. As shown in
Figure 1, the inductor stores energy during switch-on time,
while the output capacitor supports the load current. Since the
current in an inductor cannot change instantaneously, the
voltage across it reverses to maintain a constant current when
the main switch turns off. The reverse voltage is clamped to
(V_O + V_D), and the inductor is discharged through the diode,
output capacitor, and load.
The energy stored and discharged to the load is proportional to
the square of the peak inductor current. At full load, the circuit
can be designed to operate in a continuous conduction mode
to reduce the peak and RMS currents in the MOSFET switch
and an inductor. Operation is in discontinuous mode at lower
loads. The constant volt-second product determines the
operating duty cycle in continuous conduction mode while the
amount of energy needed to be stored in the inductor
determines the duty cycle in discontinuous conduction mode.
The peak and average inductor currents for continuous mode
operation are:
I_L(AVG)
+
I_OUT
Ǹ_1–d
(1)
I_L(PK)
+
I_OUT
Ǹ_1–d
)
DI_L
2
(2)
COM
I_L
Si9121
L
C
I_D
COM
∆I_L
I_L
I_L(PK)
I_L(AVG)
t
+V_OUT
SW
I_S
t
I_SW
–V_IN
I_OUT
I_D
t
FIGURE 1. Buck-Boost Converter
Document Number: 71156
04-May-00
www.vishay.com S FaxBack 408-970-5600
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