starting from the upper secondary end flows through CR1, charges capacitor C, and returns through the load

impedance RL back to the center-tap.

When Q2 conducts, Q1 goes high-impedance and the voltage polarities at the primary and secondary reverse.

Now the lower end of the primary presents the open end with a 2×VIN potential against ground. In this case CR2

is forward biased while CR1 is reverse biased and current flows from the lower secondary end through CR2,

charging the capacitor and returning through the load to the center-tap.

7.3.2 Core Magnetization

Figure 7-2 shows the ideal magnetizing curve for a push-pull converter with B as the magnetic flux density and H

as the magnetic field strength. When Q1 conducts the magnetic flux is pushed from A to A’, and when Q2

conducts the flux is pulled back from A’ to A. The difference in flux and thus in flux density is proportional to the

product of the primary voltage, VP, and the time, tON, it is applied to the primary: B ≈ VP × tON.

R

DS

B

H

A

A’

VIN

VP

V

DS

V

IN = VP + VDS

Figure 7-2. Core Magnetization and Self-Regulation Through Positive Temperature Coefficient of RDS(on)

This volt-seconds (V-t) product is important as it determines the core magnetization during each switching cycle.

If the V-t products of both phases are not identical, an imbalance in flux density swing results with an offset from

the origin of the B-H curve. If balance is not restored, the offset increases with each following cycle and the

transformer slowly creeps toward the saturation region.

Fortunately, due to the positive temperature coefficient of a MOSFET’s on-resistance, the output FETs of the

SN6501 have a self-correcting effect on V-t imbalance. In the case of a slightly longer on-time, the prolonged

current flow through a FET gradually heats the transistor which leads to an increase in RDS-on. The higher

resistance then causes the drain-source voltage, VDS, to rise. Because the voltage at the primary is the

difference between the constant input voltage, VIN, and the voltage drop across the MOSFET, VP = VIN – VDS, VP

is gradually reduced and V-t balance restored.

7.4 Device Functional Modes

The functional modes of the SN6501 are divided into start-up, operating, and off-mode.

7.4.1 Start-Up Mode

When the supply voltage at Vcc ramps up to 2.4 V typical, the internal oscillator starts operating at a start

frequency of 300 kHz. The output stage begins switching but the amplitude of the drain signals at D1 and D2 has

not reached its full maximum yet.

7.4.2 Operating Mode

When the device supply has reached its nominal value ±10% the oscillator is fully operating. However variations

over supply voltage and operating temperature can vary the switching frequencies at D1 and D2 between 250

kHz and 550 kHz for VCC = 3.3 V ±10%, and between 300 kHz and 620 kHz for VCC = 5 V ±10%.

7.4.3 Off-Mode

The SN6501 is deactivated by reducing VCC to 0 V. In this state both drain outputs, D1 and D2, are high-

impedance.

SN6501

SLLSEA0I – FEBRUARY 2012 – REVISED JANUARY 2021

www.ti.com

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