SI9130_11 Datasheet, PDF (11/15 Pages) Vishay Siliconix – Pin-Programmable Dual Controller - Portable PCs

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 µs.
Soft start helps prevent current spikes at turn-on and allows
separate supplies to be delayed using external
programmability.
Synchronous Rectifiers
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.
The low-side rectifier is shut off when the inductor current
drops to zero.
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.
Si9130
Vishay Siliconix
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.
V_L
V_L
BST_
PWM
Level
Translator
DH_
LX_
V_L
DL_
BATTERY
INPUT
Figure 4. Boost Supply for Gate Drivers
Document Number: 70190
www.vishay.com
S11-0975-Rev. G, 16-May-11
11
This document is subject to change without notice.
THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000