SC418ULTRT Semtech, SC418ULTRT Datasheet - Page 14

• Manufacturer Part Number: SC418ULTRT
• Description: IC BUCK SYNC ADJ 20MLPQ
• Manufacturer: Semtech
• Series: EcoSpeed™, SmartDrive™r
• Type: Step-Down (Buck)r

Specifications of SC418ULTRT

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 0.5 ~ 5.5 V
• Frequency - Switching: 200kHz ~ 1MHz
• Voltage - Input: 3 ~ 28 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 20-MLPQ
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -
• Other names: SC418ULTRTTR

Applications Information (continued)
comparator and a capacitor. One comparator input is con-
nected to V
capacitor. When the on-time begins, the internal capaci-
tor charges from zero volts through a current which is
proportional to V
V
turns off.
This method automatically produces an on-time that is
proportional to V
Under steady-state conditions, the switching frequency
can be determined from the on-time by the following
equation.
The SC418 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 1MHz using a resistor between the TON pin and ground.
The resistor value is selected by the following equation.
The maximum R
ing equation.
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:
FB
R
500mV
V
V
OUT
TON
OUT
IN
•
, the on-time is completed and the high-side MOSFET
f
R
SW
FB Comparator
R
VFB falls below the 500mV reference
TON
TON
On-time = K x R
-
+
T
_
One-Shot
ON
MAX
T (
V
Timer
Figure 2 — On-Time Generation
OUT
OUT
ON
25
V
, the other input is connected to the
IN
pF
TON
V
15
10
IN
IN
OUT
. When the capacitor voltage reaches
value allowed is shown by the follow-
_
ns
MIN
V
A
and inversely proportional to V
)
OUT
TON
Drives
V
Gate
x (V
IN
DH
DL
OUT
/V
V
IN
IN
V
)
Q2
Q1
LX
C
ESR
OUT
L
+
V
OUT
FB
IN
.
TON Limitations and VDDA Supply Voltage
For VDDA below 4.5V, the TON accuracy may be limited
by V
fies the below relation over the entire V
If V
range, the previous RTON equation is not accurate. In all
cases where VIN > ((VDDA - 1.6V) x 10), the RTON equation
must be modifi ed as follows.
Note that when V
actual on-time is fixed and does not vary with V
operating in this condition, the switching frequency will
vary inversely with V
frequency.
When a large capacitor is placed in parallel with R1 (CTOP)
VOUT is shown by the following equation.
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 SC418 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
V
•
IN
OUT
V
IN
R
exceeds ((VDDA - 1.6V) x 10) for all or part of the V
IN
The Zero Cross Detector trips if power-save is
active
. The previous RTON equation is accurate if V
TON
< (VDDA - 1.6V) x 10
0
5 .
(T
ON
1
IN
R
R
10ns)
is greater than ((VDDA - 1.6V) x 10), the
1
2
25pF
IN
rather than approximating fixed
V
(V5V
RIPPLE
2
V
OUT
1.6V)
1
1
IN
10
R
R
range:
2
2
R (
1
R
R
1
1
C
TOP
IN
C
. When
IN
TOP
)
2
satis-
2
14
IN