8.2 Typical Application

Figure 8-3. Typical Application Schematic (SN6501)

8.2.1 Design Requirements

For this design example, use the parameters listed in Table 8-1 as design parameters.

Table 8-1. Design Parameters

DESIGN PARAMETER

EXAMPLE VALUE

Input voltage range

3.3 V ± 3%

Output voltage

5 V

Maximum load current

100 mA

8.2.2 Detailed Design Procedure

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 5-11 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 8-7. The measured VOUT and efficiency characteristics for the

regulated and unregulated outputs are shown in Figure 5-1 to Figure 5-28.

8.2.2.1 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.

Unlike SN6505 devices, SN6501 does not have soft-start, internal current limit, or thermal shutdown (TSD)

features. Therefore to address possible unregulated or large currents, there is a limit to the maximum capacitive

load that can be connected to an SN6501 system. Loads exceeding 5uF appear as short circuits to SN6501

during power up and may affect the device’s long-term reliability. When using SN6501, it is recommended to

keep capacitive loads below 5uF or incorporate LDOs with low short-circuit current limits or soft-start features to

ensure excessive current is not drawn from SN6501.

8.2.2.2 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

SN6501

SLLSEA0I – FEBRUARY 2012 – REVISED JANUARY 2021

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