SI9161 Datasheet, PDF (9/12 Pages) Vishay Siliconix – Optimized-Efficiency Controller for RF Power Amplifier Boost Converter

Si9161
Vishay Siliconix
Startup
Designed to operate with single cell Lithium Ion battery
voltage, the Si9161 has an operating range of 2.7 V to 6.0 V.
During start-up, the device requires 3.0 V to guarantee proper
operation, although it will typically start up at less than 2.2 V.
Once powered, Si9161 will continue to operate until the
voltage at V_DD is 2.7 V; at this point, the battery is basically
dead. During start-up, power for the chip is provided by the
battery through R5 to V_DD and through schottky diode D1 to
V_S pins. Once the converter is fully operating, V_S supply
power is provided by the converter output through diode D2,
which overrides the D1 diode. This self perpetuating method
of powering further improves the converter efficiency by
utilizing higher gate drive to lower the on-resistance loss of
the MOSFET.
Another benefit of powering from the output voltage is it
provides minimum load on the converter. This prevents the
converter from skipping frequency pulses typically referred to
as Burst or Pulse-Skipping modes. Pulse skipping mode could
be dangerous, especially if it generates noise in RF, IF, or
signal processing frequency bands.
Enable and Under Voltage Shutdown
The Si9161 is designed with programmable under-voltage
lockout and enable features. These features give designers
flexibility to customize the converter design. The under-
voltage lockout threshold is 1.2 V. With a simple resistor
divider from V_DD, Si9161 can be programmed to turn-on at
any V_DD voltage. The ENABLE pin, a TTL logic compatible
input, allows remote shutdown as needed.
FIGURE 2. Gate Drive Timing Diagrams
The MOSFET used is the Si6801, an n- and p-channel in a
single package TSSOP-8. The Si6801 is optimized to have
very low gate charge and gate resistance. This results in a
great reduction in gate switching power losses. The average
time to switch on and off a MOSFET in a conventional
structure is about 20 ns. The Si6801 will switch on and off in
<5 ns, see Figure 3.
Gate Drive and MOSFETs
The gate drive section is designed to drive the high-side
p-channel switch and low-side n-channel switch. The internal
40 ns break-before-make (BBM) timing prevents both
MOSFETs from turning-on simultaneously. The BBM circuit
monitors both drive voltages, once the gate-to-source voltage
drops below 2.5 V, the other gate drive is delayed 40-ns
before it is allowed to drive the external MOSFET (see
Figure 2 for timing diagram). This smart gate drive control
provides additional assurance that shoot-through current will
not occur.
Note the Speed
These MOSFETs have switching speeds of <5 ns. This high speed is
due to the fast, high current output drive of the Si9161 and the
optimized gate charge of the Si6801.
FIGURE 3. Gate Switching Times
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S-60752—Rev. B, 05-Apr-99
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