SI9130 Datasheet, PDF (12/13 Pages) Vishay Siliconix – Pin-Programmable Dual Controller-Portable PCs

Si9130
Vishay Siliconix
OPERATIONAL MODES
PWM Mode
The 3.3-V and 5-V Buck controllers operate in continuous-current
PWM mode when the load demands more than approximately
25% of the maximum current (see typical curves). The duty cycle
can be approximated as Duty_Cycle = V_OUT/V_IN.
In this mode, the inductor current is continuous; in the first half of
the cycle, the current slopes up when the high-side MOSFET
conducts and then, in the second half, slopes back down when
the inductor is providing energy to the output capacitor and load.
As current enters the inductor in the first half-cycle, it is also
continuing through to the load; hence, the load is receiving
continuous current from the inductor. By using this method, output
ripple is minimized and smaller form-factor inductors can be used.
The output capacitor’s ESR has the largest effect on output ripple.
It is typically under 50mV; the worst case condition is under light
load with higher input battery voltage.
Pulse-Skipping Mode
The SYNC pin can be driven with an external CMOS level signal
with frequency from 240 kHz and 350 kHz to synchronize to the
internal oscillator. Tying SYNC to either V_L or GND sets the
frequency to 200 kHz and to REF sets the frequency to 300 kHz.
Operation at 300 kHz is typically used to minimize output passive
component sizes. Slower switching speeds of 200 kHz may be
needed for lower input voltages.
Internal V_L and REF
A 5-V linear regulator supplies power to the internal logic circuitry.
The regulator is available for external use from pin V_L , able to
source 5 mA. A 4.7-mF capacitor should be connected between
V_L and GND. To increase efficiency, when the 5-V switching
supply has voltage greater than 4.5 V, V_L is internally switched
over to the output of the 5-V switching supply and the linear
regulator is turned off.
When the load requires less than 25% of its maximum, the
Si9130 enters a mode which drives the gate for one clock cycle
and skips the majority of the remaining cycles. Pulse-skipping
mode cuts down on the switching losses, the dominant power
consumer at low current levels.
In the region between pulse-skipping mode and PWM mode, the
controller may transition between the two modes, delivering
spurts of pulses. This may cause the current waveform to look
irregular, but will not overly affect the ripple voltage. Even in this
transitioning mode efficiency will stay high.
Current Limit
The 5-V linear regulator provides power to the internal 3.3-V
bandgap reference (REF). The 3.3-V reference can supply 5 mA
to an external load, connected to pin REF. Between REF and
GND connect a capacitor, 0.22 mF plus 1 mF per mA of load
current. The switching outputs will vary with the reference;
therefore, placing a load on the REF pin will cause the main
outputs to decrease slightly, within the specified regulation
tolerance.
V_L and REF supplies stay on as long as V+ is greater than 4.5
V, even if the switching supplies are not enabled. This feature is
necessary when using the micropower regulators to keep
memory alive during shutdown.
The current through an external resistor, is constantly monitored
to protect against over-current. A low value resistor is placed in
series with the inductor. The voltage across it is measured by
connecting it between CS_ and FB_. If this voltage is larger than
100 mV, the high-side MOSFET drive is shut down. Eliminating
over-currents protects the MOSFET, the load and the power
source. Typical values for the sense resistors with a 3-A load will
be 25 mW.
Oscillator and SYNC
There are two ways to set the Si9130 oscillator frequency: by
using an external SYNC signal, or using the internal oscillator.
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Both linear regulators can be connected to their respective
switching supply outputs. For example, REF would be tied to the
output of the 3.3 V and V_L to 5 V. This will keep the main supplies
up in standby mode, provided that each load current in shutdown
is not larger than 5 mA.
Fault Protection
The 3.3-V and 5-V switching controllers are shut down when one
of the linear regulators drops below 85% of its nominal value; that
is, shut down will occur when V_L < 4.0 V or REF < 2.8 V.
Document Number: 70190
S-40805—Rev. F, 26-Apr-04