SI786 Datasheet, PDF (12/14 Pages) Vishay Siliconix – Dual-Output Power-Supply Controller

Si786
Vishay Siliconix
V_L
PWM
Level
Translator
V_L
BST_
DH_
LX_
V_L
DL_
BATTERY
INPUT
FIGURE 4. Boost Supply for Gate Drivers
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 50 mV; the
worst case condition is under light load with higher input
battery voltage.
Pulse-Skipping Mode
When the load requires less than 25% of its maximum, the
Si786 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 transitional mode efficiency will stay high.
Current Limit
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.
www.vishay.com
12
Document Number: 70189
S-40807—Rev. J, 26-Apr-04