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Document Number: 69062
S10-1367-Rev. D, 14-Jun-10
Vishay Siliconix
SiC417
On-Time One-Shot Generator (t
ON
) and Operating
Frequency
The SiC417 have an internal on-time one-shot generator
which is a comparator that has two inputs. The FB
Comparator output goes high when VFB is less than the
internal 500 mV reference. This feeds into the gate drive and
turns on the high-side MOSFET, and also starts the one-shot
timer. The one-shot timer uses an internal comparator and a
capacitor. One comparator input is connected to V
OUT
, the
other input is connected to the capacitor. When the on-time
begins, the internal capacitor charges from zero volts
through a current which is proportional to V
IN
. When the
capacitor voltage reaches V
OUT
, the on-time is completed
and the high-side MOSFET turns off. The figure 2 shows the
on-chip implementation of on-time generation.
This method automatically produces an on-time that is
proportional to V
OUT
and inversely proportional to V
IN
. Under
steady-state conditions, the switching frequency can be
determined from the on-time by the following equation.
The SiC417 uses an external resistor to set the ontime which
indirectly sets the frequency. The on-time can be
programmed to provide operating frequency from 200 kHz to
1 MHz using a resistor between the t
ON
pin and ground. The
resistor value is selected by the following equation.
The maximum R
TON
value allowed is shown by the following
equation.
V
OUT
Voltage Selection
The switcher output voltage is regulated by comparing V
OUT
as seen through a resistor divider at the FB pin to the internal
500 mV reference voltage, see figure 3.
As the control method regulates the valley of the output ripple
voltage, the DC output voltage V
OUT
is off set by the output
ripple according to the following equation.
V
OUT
= 0.5 x (1 + R
1
/R
2
) + V
RIPPLE
/2
Enable and Power-Save Inputs
The EN/PSV and ENL inputs are used to enable or disable
the switching regulator and the LDO.
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 ?/DIV>
internal resistor via the V
OUT
pin.
When EN/PSV is allowed to float, the pin voltage will fl oat to
1.5 V. The switching regulator turns on with power-save
disabled and all switching is in forced continuous mode.
When EN/PSV is high (above 2.0 V), the switching regulator
turns on with ultra-sonic power-save enabled. The SiC417
ultra-sonic power-save operation maintains a minimum
switching frequency of 25 kHz, for applications with stringent
audio requirements.
The ENL input is used to control the internal LDO. This input
serves a second function by acting as a V
IN UVLO
sensor for
the switching regulator.
The LDO is off when ENL is low (grounded). When ENL is a
logic high but below the V
IN UVLO
threshold (2.6 V typical),
then the LDO is on and the switcher is off. When ENL is
above the V
IN UVLO
threshold, the LDO is enabled and the
switcher is also enabled if the EN/PSV pin is not grounded.
Forced Continuous Mode Operation
The SiC417 operates the switcher in Forced Continuous
Mode (FCM) 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.
Figure 2 - On-Time Generation
FB
500 mV
-
+
V
OUT
V
IN
R
ton
On-time = K x R
ton
x (V
OUT/
V
IN
)
FB comparator
One-shot
timer
Gate
drives
DH
DL
Q1
Q2
L
Q1
ESR
FB
V
OUT
C
OUT
V
LX
+
f
SW
=
V
OUT
t
ON
x V
IN
R
ton
=
(t
ON
- 10 ns) x V
IN
25 pF x V
OUT
R
ton_MAX
=
V
IN_MIN
15 礎
Figure 3 - Output Voltage Selection
V
OUT
R
1
R
2
To FB pin
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