SC424 SEMTECH [Semtech Corporation], SC424 Datasheet - Page 14

• Manufacturer Part Number: SC424
• Description: 6A Integrated FET Regulator with 5V LDO
• Manufacturer: SEMTECH [Semtech Corporation]

Applications Information
Synchronous Buck Converter
The SC414/SC424 is a step down synchronous DC-DC buck
converter with integrated power MOSFETs and a 5V LDO.
The device is capable of 6A operation at very high effi -
ciency. A space saving 4x4 (mm) 28-pin package is used.
The programmable operating frequency range of 200kHz
to 1MHz enables optimizing the configuration for PCB
area and effi ciency.
The buck controller uses a pseudo-fi xed frequency adap-
tive on-time control. This control method allows fast tran-
sient response which permits the use of smaller output
capacitors.
Input Voltage Requirements
The SC414/SC424 requires two input supplies for normal
operation: V
from 3V to 28V. V5V requires a 3.3 or 5V supply input that
can be an external source or the internal LDO confi gured
to supply 5V. If the LDO is enabled, V5V voltage must be
> 5V.
Psuedo-fi xed Frequency Adaptive On-time Control
The PWM control method used by the SC414/SC424 is
pseudo-fi xed frequency, adaptive on-time, as shown in
Figure 1. The ripple voltage generated at the output
capacitor (ESR) is used as a PWM ramp signal. This ripple is
used to trigger the on-time of the controller.
Q1
Q2
V
IN
Figure 1 — PWM Control Method, V
V
LX
IN
and V5V. V
C
IN
L
TON
IN
operates over the wide range
V
+
ESR
FB
C
OUT
V
LX
OUT
FB Threshold
Ripple
FB
V
OUT
The adaptive on-time is determined by an internal one-
shot timer. When the one-shot is triggered by the output
ripple, the device sends a single on-time pulse to the high-
side MOSFET. The pulse period is determined by V
V
inversely proportional to input voltage. With this adaptive
on-time confi guration, the device automatically antici-
pates the on-time needed to regulate V
V
The advantages of adaptive on-time control are:
One-Shot Timer and Operating Frequency
One-shot timer operation is shown in Figure 2. The FB
Comparator output goes high when V
internal 750mV reference. This feeds into the gate drive
and turns on the high-side MOSFET, and starts the one-
shot timer. The one-shot timer uses an internal compara-
tor and a capacitor. One comparator input is connected to
V
the on-time begins, the internal capacitor charges from
zero volts through a current which is proportional to V
When the capacitor voltage reaches V
completed and the high-side MOSFET turns off .
FB
R
750mV
V
V
IN
IN
OUT
•
•
•
•
•
OUT
IN
TON
. The period is proportional to output voltage and
condition and at the selected frequency.
, the other input is connected to the capacitor. When
FB Comparator
Predictable operating frequency compared to
other variable frequency methods.
Reduced component count by eliminating the
error amplifi er and compensation components.
Reduced component count by removing the
need to sense and control inductor current.
Fast transient response — the response time is
controlled by a fast comparator instead of a typi-
cally slow error amplifi er.
Reduced output capacitance due to fast tran-
sient response
On-time = K x R
-
+
One-Shot
Figure 2 — On-Time Generation
Timer
TON
Drives
Gate
x (V
DH
DL
OUT
/V
SC414/SC424
V
IN
IN
V
)
Q2
Q1
LX
OUT
OUT
FB
C
ESR
OUT
L
, the on-time is
is less than the
for the present
+
OUT
V
OUT
FB
and
14
IN
.