SI786 Datasheet, PDF (9/14 Pages) Vishay Siliconix – Dual-Output Power-Supply Controller

Si786
Vishay Siliconix
3.3-V and 5-V Switching Controllers
Each PWM controller on the Si786 is identical with the
exception of the preset output voltages. The controllers only
share three functional blocks (see Figure 2): the oscillator, the
voltage reference (REF) and the 5-V logic supply (V_L). The
3.3-V and 5-V controllers are independently enabled with pins
ON₃ and ON₅ , respectively. The PWMs are a direct-summing
type, without the typical integrating error amplifier along with
the phase shift which is a side effect of this type of topology.
Feedback compensation is not needed, as long as the output
capacitance and its ESR requirements are met, according to
the Design Considerations section of this data sheet.
The main PWM comparator is an open loop device which is
comprised of three comparators summing four signals: the
feedback voltage error signal, current sense signal,
slope-compensation ramp and voltage reference as shown in
Figure 3. This method of control comes closer to the ideal of
maintaining the output voltage on a cycle-by-cycle basis.
When the load demands high current levels, the controller is in
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-mA 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
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
Document Number: 70189
S-40807—Rev. J, 26-Apr-04
full PWM mode. Every cycle from the oscillator asserts the
output latch and drives the gate of the high-side MOSFET for
a period determined by the duty cycle (approximately
V_OUT/V_IN 100%) and the frequency. The high-side switch
turns off, setting the synchronous rectifier latch and 60ns 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.
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 Si786 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 Si786 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.
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