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SC416

Applications Information

SC416 Synchronous Buck Controller

The SC416 is a dual synchronous controller which sim-

plifies the task of designing a dual-output power sup-

ply with synchronized or tracking ramps for startup and

shutdown.

VIN and +5V Bias Supplies

The SC416 requires an external +5V bias supply in ad-

dition to the VIN supply. If stand-alone capability is re-

quired, the +5V supply can be generated with an exter-

nal 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 ca-

pacitor’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 proportion-

al to input voltage. A separate one-shot sets the mini-

mum off-time (typically 330ns). The two converters are

designed to operate at different frequencies to reduce

interaction; side2 frequency is set typically 20% higher

than side1.

COUT

VIN

ESR

CIN

VOUT

FB Threshold

750mV

VFB

VLX

TON

Figure 1

On-Time One-Shot (TON)

Each internal on-time one-shot comparator has two in-

puts. 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 cur-

rent. 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 in-

versely proportional to input voltage. This implementa-

tion results in a fairly constant switching frequency with

no clock generator.

The nominal frequency is set through an external resis-

tor RTON 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 gen-

eral equations for the side1 and side2 on-times are:

TON1 = 3.30 × (RTON + 37) × (VOUT/VIN) + 35

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

(TON in nsec, RTON in kΩ)

Switch-Mode Operation

The switch-mode operation is explained below, and is

identical for both sides except for the TON timing.

The output voltage is sensed at the FB pin and is com-

pared to the internal 750mV reference. (The output volt-

age 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 trig-

gers 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.

Once high, DL has a minimum pulse width of typically

330nsec which is the minimum off-time. At the end of

the minimum off-time, DL continues to stay high until

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