SN6501DBVT datasheet(15/29 Pages) TI1

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SN6501

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SLLSEA0E – FEBRUARY 2012 – REVISED JANUARY 2013

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.

CONVERTER DESIGN

The following recommendations on components selection focus on the design of an efficient push-pull converter

with high current drive capability. Contrary to popular belief, the output voltage of the unregulated converter

output drops significantly over a wide range in load current. The characteristic curve in Figure 25 for example

shows that the difference between VOUT at minimum load and VOUT at maximum load exceeds a transceiver’s

supply range. Therefore, in order to provide a stable, load independent supply while maintaining maximum

possible efficiency the implementation of a low dropout regulator (LDO) is strongly advised.

The final converter circuit is shown in Figure 52. The measured VOUT and efficiency characteristics for the

regulated and unregulated outputs are shown in Figure 21 to Figure 20.

SN6501 DRIVE CAPABILITY

The SN6501 transformer driver is designed for low-power push-pull converters with input and output voltages in

the range of 3 V to 5.5 V. While converter designs with higher output voltages are possible, care must be taken

that higher turns ratios don’t lead to primary currents that exceed the SN6501 specified current limits.

LDO SELECTION

The minimum requirements for a suitable low dropout regulator are:

•

Its current drive capability should slightly exceed the specified load current of the application to prevent the

LDO from dropping out of regulation. Therefore for a load current of 100 mA, choose a 100 mA to 150 mA

LDO. While regulators with higher drive capabilities are acceptable, they also usually possess higher dropout

voltages that will reduce overall converter efficiency.

•

The internal dropout voltage, VDO, at the specified load current should be as low as possible to maintain

efficiency. For a low-cost 150 mA LDO, a VDO of 150 mV at 100 mA is common. Be aware however, that this

lower value is usually specified at room temperature and can increase by a factor of 2 over temperature,

which in turn will raise the required minimum input voltage.

•

The required minimum input voltage preventing the regulator from dropping out of line regulation is given with:

VI-min = VDO-max + VO-max.

This means in order to determine VI for worst-case condition, the user must take the maximum values for VDO

and VO specified in the LDO data sheet for rated output current (i.e., 100 mA) and add them together. Also

specify that the output voltage of the push-pull rectifier at the specified load current is equal or higher than VI-

min. If it is not, the LDO will lose line-regulation and any variations at the input will pass straight through to the

output. Hence below VI-min the output voltage will follow the input and the regulator behaves like a simple

conductor.

•

The maximum regulator input voltage must be higher than the rectifier output under no-load. Under this

condition there is no secondary current reflected back to the primary, thus making the voltage drop across

RDS-on negligible and allowing the entire converter input voltage to drop across the primary. At this point the

secondary reaches its maximum voltage of

VS-max = VIN-max × n

with VIN-max as the maximum converter input voltage and n as the transformer turns ratio. Thus to prevent the

LDO from damage the maximum regulator input voltage must be higher than VS-max. Table 1 lists the maximum

secondary voltages for various turns ratios commonly applied in push-pull converters with 100 mA output drive.

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