SIC403A_13 Datasheet, PDF (13/25 Pages) Vishay Siliconix – 6 A microBUCK SiC403A/B Integrated Buck Regulator with Programmable LDO

When a large capacitor is placed in parallel with R1 (C_TOP)
V_OUT is shown by the following equation.
V_OUT = 0.6 x
1+
R₁
R₂
+ ^VRIPPLE
2
x
1 + (R1ωCTOP)²
1+
R₂
R₂
x
+
R₁
R₁
ωCTOP
2
Enable and Power-Save Inputs
The EN/PSV input is used to enable or disable the switching
regulator. When EN/PSV is low (grounded), the switching
regulator is off and in its lowest power state. When off, the
output of the switching regulator soft-discharges the output
into a 15  internal resistor via the V_OUT pin. When EN/PSV
is allowed to float, the pin voltage will float to 33 % of the
voltage at V_DD. The switching regulator turns on with
power-save disabled and all switching is in forced continuous
mode.
When EN/PSV is high (above 44 % of the voltage at V_DD),
the switching regulator turns on with power-save enabled.
The SiC403A/B P-Save operation reduces the switching
frequency according to the load for increased efficiency at
light load conditions.
Forced Continuous Mode Operation
The SiC403A/B operates the switcher in FCM (Forced
Continuous Mode) by floating the EN/PSV pin (see figure 4).
In this mode one of the power MOSFETs is always on, with
no intentional dead time other than to avoid cross-
conduction. This feature results in uniform frequency across
the full load range with the trade-off being poor efficiency at
light loads due to the high-frequency switching of the
MOSFETs. DH is gate signal to drive upper MOSFET. DL is
lower gate signal to drive lower MOSFET
FB ripple
voltage (VFB)
FB threshold
(600 mV)
Inductor
current
DC load current
On-time
(tON)
DH on-time is triggered when
VFB reaches the FB threshold
DH
DL
DL drives high when on-time is completed.
DL remains high until VFB falls to the FB threshold.
Figure 4 - Forced Continuous Mode Operation
SiC403A, SiC403BCD
Vishay Siliconix
Ultrasonic Power-Save Operation (SiC403A)
The SiC403A provides ultrasonic power-save operation at
light loads, with the minimum operating frequency fixed at
slightly under 25 kHz. This is accomplished by using an
internal timer that monitors the time between consecutive
high-side gate pulses. If the time exceeds 40 µs, DL drives
high to turn the low-side MOSFET on. This draws current
from V_OUT through the inductor, forcing both V_OUT and V_FB
to fall. When V_FB drops to the 600 mV threshold, the next DH
(the drive signal for the high side FET) on-time is triggered.
After the on-time is completed the high-side MOSFET is
turned off and the low-side MOSFET turns on. The low-side
MOSFET remains on until the inductor current ramps down
to zero, at which point the low-side MOSFET is turned off.
Because the on-times are forced to occur at intervals no
greater than 40 µs, the frequency will not fall far below
25 kHz. Figure 5 shows ultrasonic power-save operation.
FB ripple
voltage (VFB)
minimum fSW _~ 25 kHz
FB threshold
(600 mV)
Inductor
(0A)
current
On-time
(tON)
DH on-time is triggered when
VFB reaches the FB threshold
DH
40 µs time-out
DL
After the 40 µs time-out, DL drives high if VFB
has not reached the FB threshold.
Figure 5 - Ultrasonic Power-Save Operation
Power-Save Operation (SiC403B)
The SiC403B provides power-save operation at light loads
with no minimum operating frequency. With power-save
enabled, the internal zero crossing comparator monitors the
inductor current via the voltage across the low-side MOSFET
during the off-time. If the inductor current falls to zero
for 8 consecutive switching cycles, the controller enters
MOSFET on each subsequent cycle provided that the
power-save operation. It will turn off the low-side MOSFET
on each subsequent cycle provided that the current crosses
zero. At this time both MOSFETs remain off until V_FB drops
to the 600 mV threshold. Because the MOSFETs are off, the
load is supplied by the output capacitor.
If the inductor current does not reach zero on any switching
cycle, the controller immediately exits power-save and
returns to forced continuous mode.
Figure 6 shows power-save operation at light loads.
Document Number: 62768
For technical questions, contact: [e-mail]
www.vishay.com
S12-1972-Rev. A, 27-Aug-12
13
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