SC418
4
comparator and a capacitor. One comparator input is con-
nected to V
OUT
, the other input is connected to the
capacitor. When the on-time begins, the internal capaci-
tor 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.
Gate
Drives
FB Comparator
One-Shot
Timer
On-time = K x R
TON
x (V
OUT
/V
IN
)
OUT
IN
B
00mV
Q1
Q2
L
C
OUT
V
IN
ESR
+
V
OUT
V
LX
FB
DH
DL
TON
+
-
Figure 2 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.
IN
ON
OUT
SW
V
T
V
f
The SC48 uses an external resistor to set the on-time which
indirectly sets the frequency. The on-time can be pro-
grammed to provide an operating frequency from 200kHz
to MHz using a resistor between the TON pin and ground.
The resistor value is selected by the following equation.
OUT
IN
ON
TON
V
pF
25
V
)
ns
10
T
(
R
The maximum R
TON
value allowed is shown by the follow-
ing equation.
A
15
V
R
MIN
_
IN
MAX
_
TON
Immediately after the on-time, the DL output drives high
to energize the low-side MOSFET. DL has a minimum high
time of ~250ns, after which DL continues to stay high until
one of the following occurs:
VFB falls below the 500mV reference
"
The Zero Cross Detector trips if power-save is
active
TON Limitations and VDDA Supply Voltage
For VDDA below 4.5V, the TON accuracy may be limited
by V
IN
. The previous RTON equation is accurate if V
IN
satis-
fes the below relation over the entire V
IN
range:
V
IN
< (VDDA - 1.6V) x 10
If V
IN
exceeds ((VDDA - .6V) x 0) for all or part of the V
IN
range, the previous RTON equation is not accurate. In all
cases where VIN > ((VDDA - .6V) x 0), the RTON equation
must be modifi ed as follows.
OUT
ON
TON
V
25pF
10
1.6V)
(V5V
10ns)
(T
R
Note that when V
IN
is greater than ((VDDA - .6V) x 0), the
actual on-time is fixed and does not vary with V
IN
. When
operating in this condition, the switching frequency will
vary inversely with V
IN
rather than approximating fixed
frequency.
When a large capacitor is placed in parallel with R (CTOP)
VOUT is shown by the following equation.
2
TOP
1
2
1
2
2
TOP
1
RIPPLE
2
1
OUT
C
R
R
R
R
1
)
C
R
(
1
2
V
R
R
1
5
.
0
V
The switcher output voltage can be programmed higher
than 5V with careful design. In this case the VOUT pin
cannot connect directly to the switcher output due to its
the maximum voltage rating. An additional resistor
divider network is required to connect from the switcher
output to the VOUT pin. The voltage at the VOUT pin
should be at least 500mV lower than the VDDA supply, to
prevent the VLDO switch-over function. For example, the
voltage at the VOUT pin can be 4V if VDDA is set for 5V.
When the SC48 operates from an external power source
and the LDO is disabled by grounding the ENL pin, the
voltage at the VOUT pin can be as high as shown in
Recommended Operating Conditions. Note that RTON
must be adjusted higher by the same divider ratio to
"
Applications Information (continued)
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