C

CR1

CR2

Q1

Q2

VIN

VOUT

RL

C

CR1

CR2

Q1

Q2

VIN

VOUT

RL

OSC

MOSFET

Driver

D1

D2

TEMP

÷ 2

UVLO

I-LIM

SN6505

SSC

GND

GND

EN

CLK

Ext CLK

Detect

10

SN6505A, SN6505B

SLLSEP9B – SEPTEMBER 2015 – REVISED FEBRUARY 2016

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Product Folder Links: SN6505A SN6505B

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8 Detailed Description

8.1 Overview

The SN6505 is a transformer driver designed for low-cost, small form-factor, isolated DC-DC converters utilizing

the push-pull topology. The device includes an oscillator that feeds a gate-drive circuit. The gate-drive,

comprising a frequency divider and a break-before-make (BBM) logic, provides two complementary output

signals which alternately turn the two output transistors on and off.

The output frequency of the oscillator is divided down by two . A subsequent break-before-make logic inserts a

dead-time between the high-pulses of the two signals. Before either one of the gates can assume logic high, the

BBM logic ensures a short time period during which both signals are low and both transistors are high-

impedance. This short period, is required to avoid shorting out both ends of the primary. The resulting output

signals, present the gate-drive signals for the output transistors.

8.2 Functional Block Diagram

8.3 Feature Description

8.3.1 Push-Pull Converter

Push-pull converters require transformers with center-taps to transfer power from the primary to the secondary

(see Figure 14).

Figure 14. Switching Cycles of a Push-Pull Converter

At the same time the voltage across the upper half of the primary is such that the upper primary end is positive

Per dot convention the same voltage polarities that occur at the primary also occur at the secondary. The