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SC415

Applications Information

SC415 Synchronous Buck Controller

The SC415 is a dual synchronous controller which simplifies

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-fixed 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 different frequencies to minimize interaction.

Side2 frequency is set typically 20% higher than Side1.

Figure 1

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:

TON1 = 3.3 × (RTON+37) × (VOUT/VIN) + 35

TON2 = 2.75 × (RTON+37) × (VOUT/VIN) + 35

(TON2 in nsec, RTON in kΩ)

Switch-Mode Operation

The switch-mode operation is explained below and is

identical for both sides except for the difference 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.

COUT

VIN

ESR

CIN

VOUT

FB Threshold

0.75V

VOUT / FB

TON

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