SC461 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information
Synchronous Buck Converter
The SC461 is a step down synchronous DC-DC buck con-
troller with an internal 5V LDO. It provides efficient opera-
tion in a space saving 3x3 (mm) 20-pin package. The
programmable operating frequency range up to 1MHz
enables optimizing the configuration for PCB area and
efficiency.
The controller uses a pseudo-fixed frequency adaptive
on-time control. This allows fast transient response which
permits the use of smaller output capacitors.
Input Voltage Requirements
The SC461 requires two input supplies for normal opera-
tion: VIN and VDDA/VDDP. VIN operates over the wide
range of 3V to 28V. VDDA and VDDP require a 3.3V or 5V
supply which can be from an external source or from the
internal LDO. VDDA and VDDP must be derived from the
same source voltage.
Psuedo-fixed Frequency Adaptive On-time Control
The PWM control method used by the SC461 is pseudo-
fixed 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.
VIN
Q1
VLX
Q2
TON
VLX
CIN
VFB
FB Threshold
L
ESR
+
COUT
VOUT
FB
Figure 1 — PWM Control Method, VOUT Ripple
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 duration is determined by VOUT
and VIN. The duration is proportional to output voltage
and inversely proportional to input voltage. With this
adaptive on-time configuration, the device automatically
anticipates the on-time needed to regulate VOUT for the
present VIN condition and at the selected frequency.
The advantages of adaptive on-time control are:
• Predictable operating frequency compared to
other variable frequency methods.
• Reduced component count by eliminating the
error amplifier 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 amplifier.
• Reduced output capacitance due to fast tran-
sient response.
One-Shot Timer and Operating Frequency
One-shot timer operation is shown in Figure 2. The FB
comparator output goes high when VFB is less than the
internal 600mV reference. This feeds into the DH 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 con-
nected to VOUT, the other input is connected to the capaci-
tor. When the on-time begins, the capacitor charges from
zero volts through a current which is proportional to VIN.
When the capacitor voltage reaches VOUT, the on-time is
completed and the high-side MOSFET turns off.
FB
FB Comparator
-
+
REF
Gate
Drives VIN
DH
Q1
VLX L
VOUT
VIN
One-Shot
Timer
ESR
DL
Q2
COUT
+
RTON On-time = K x RTON x (VOUT/VIN)
VOUT
FB
Figure 2 — On-Time Generation
13
Share Link: 
