SC415 Semtech Corporation, SC415 Datasheet - Page 9

• Manufacturer Part Number: SC415
• Description: Dual Synchronous Buck Controller
• Manufacturer: Semtech Corporation

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Applications Information
SC415 Synchronous Buck Controller
The SC415 is a dual synchronous controller which simplifi es
the task of designing a dual-output power supply.
VIN and +5V Bias Supplies
The SC415 requires an external +5V bias supply in addition
to the VIN supply. If stand-alone capability is required,
the +5V supply can be generated with an external linear
regulator.
Pseudo-Fixed Frequency Constant On-Time PWM
Controller
The PWM control method for each output is a constant-on-
time, pseudo-fi xed frequency PWM controller, see Figure 1.
The ripple voltage seen across the output capacitor’s ESR
provides the PWM ramp signal. The on-time is determined
by an internal one-shot whose period is proportional
to output voltage and inversely proportional to input
voltage. A separate one-shot sets the minimum off -time
(typically 330ns). The two converters are designed to
operate at diff erent frequencies to minimize interaction.
Side2 frequency is set typically 20% higher than Side1.
Q1
Q2
VIN
LX
C
IN
L
Figure 1
TON
V
OUT
+
ESR
/ FB
C
OUT
LX
V
OUT
FB Threshold
0.75V
On-Time One-Shot (TON)
Each side has an internal on-time one-shot comparator
which has two inputs. One input looks at the output
voltage via the VOUT pin, while the other input samples
the input voltage via the TON pin and converts it to a
proportional current. This current charges an internal on-
time capacitor. The TON on-time is the time required for
this capacitor to charge from zero volts to VOUT, thereby
making the on-time directly proportional to output
voltage and inversely proportional to input voltage. This
implementation results in a fairly constant switching
frequency with no clock generator.
The nominal frequency is set through an external resistor
connected between VIN and the TON pin. To minimize
interaction between the two converters, Side2 is set
to operate at a slightly higher frequency. The general
equations for the side1 and side2 on-times are:
Switch-Mode Operation
The switch-mode operation is explained below and is
identical for both sides except for the diff erence in the
TON timing.
The output voltage is sensed at the FB pin and is compared
to the internal 750mV reference. (The output voltage can
also be sensed at the VOUT pin which uses an internal
resistor divider, see VOUT Voltage Selection.) When the
sensed voltage drops below 750mV, this triggers a single
TON pulse, which is fed to the DH high-side driver. The DH
pulse-width follows TON according to the TON equation,
and after that time DH drives low to shut off the high-side
MOSFET.
After DH drives low, the DL output drives high to energize
the low-side MOSFET.
TON1 = 3.3 × (RTON+37) × (VOUT/VIN) + 35
TON2 = 2.75 × (RTON+37) × (VOUT/VIN) + 35
(TON2 in nsec, RTON in kΩ)
SC415
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