SI9130 Datasheet, PDF (11/13 Pages) Vishay Siliconix – Pin-Programmable Dual Controller-Portable PCs

Si9130
Vishay Siliconix
The high-side switch turns off, setting the synchronous rectifier
latch and 60 ns later, the rectifier MOSFET turns on. The low-side
switch stays on until the start of the next clock cycle in continuous
mode, or until the inductor current becomes positive again, in
discontinuous mode. In over-current situations, where the
inductor current is greater than the 100-mV current-limit threshold,
the high-side latch is reset and the high-side gate drive is shut off.
During low-current load requirements, the inductor current will not
deliver the 25-mV minimum current threshold. The Minimum
Current comparator signals the PWM to enter pulse-skipping
mode when the threshold has not been reached. pulse-skipping
mode skips pulses to reduce switching losses, the losses which
decrease efficiency the most at light load. Entering this mode
causes the minimum current comparator to reset the high-side
latch at the beginning of each oscillator cycle.
Soft-Start
To slowly bring up the 3.3-V and 5-V supplies, connect capacitors
from SS₃ and SS₅ to GND. Asserting ON₃ or ON₅ starts a 4-A
constant current source to charge these capacitors to 4 V. As the
voltage on these pins ramps up, so does the current limit
comparator threshold, to increase the duty cycle of the MOSFETs
to their maximum level. If ON₃ or ON₅ are left low, the respective
capacitor is discharged to GND. Leaving the SS₃ or SS₅ pins
open will cause either controller to reach the terminal over-current
level within 10 ms.
Soft start helps prevent current spikes at turn-on and allows
separate supplies to be delayed using external programmability.
Synchronous Rectifiers
Shoot-through current is the result when both the high-side and
rectifying MOSFETs are turned on at the same time.
Break-before-make timing internal to the Si9130 manages this
potential problem. During the time when neither MOSFET is on,
the Schottky is conducting, so that the body diode in the low-side
MOSFET is not forced to conduct.
Synchronous rectification is always active when the Si9130 is
powered-up, regardless of the operational mode.
Gate-Driver Boost
The high-side n-channel drive is supplied by a flying-capacitor
boost circuit (see Figure 4). The capacitor takes a charge from
V_L and then is connected from gate to source of the high-side
MOSFET to provide gate enhancement. At power-up, the
low-side MOSFET pulls LX_ down to GND and charges the
BST_ capacitor connected to 5 V. During the second half of the
oscillator cycle, the controller drives the gate of the high-side
MOSFET by internally connecting node BST_ to DH_. This
supplies a voltage 5 V higher than the battery voltage to the gate
of the high-side MOSFET.
Oscillations on the gates of the high-side MOSFET in
discontinuous mode are a natural occurrence caused by the LC
network formed by the inductor and stray capacitance at the LX_
pins. The negative side of the BST_ capacitor is connected to the
LX_ node, so ringing at the inductor is translated through to the
gate drive.
BATTERY
INPUT
Synchronous rectification replaces the Schottky rectifier with a
MOSFET, which can be controlled to increase the efficiency of the
circuit.
When the high-side MOSFET is switched off, the inductor will try
to maintain its current flow, inverting the inductor’s polarity. The
path of current then becomes the circuit made of the Schottky
diode, inductor and load, which will charge the output capacitor.
The diode has a 0.5-V forward voltage drop, which contributes a
significant amount of power loss, decreasing efficiency. A
low-side switch is placed in parallel with the Schottky diode and
is turned on just after the diode begins to conduct. Because the
r_DS(ON) of the MOSFET is low, the I*R voltage drop will not be as
large as the diode, which increases efficiency.
V_L
V_L
BST_
PWM
Level
Translator
DH_
LX_
V_L
DL_
The low-side rectifier is shut off when the inductor current drops
to zero.
Document Number: 70190
S-40805—Rev. F, 26-Apr-04
FIGURE 4. Boost Supply for Gate Drivers
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