Datasheet

SN6501
www.ti.com SLLSEA0F –FEBRUARY 2012–REVISED AUGUST 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 R
DS-on
. The higher
resistance then causes the drain-source voltage, V
DS
, to rise. Because the voltage at the primary is the
difference between the constant input voltage, V
IN
, and the voltage drop across the MOSFET, V
P
= V
IN
– V
DS
, V
P
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 41 for example
shows that the difference between V
OUT
at minimum load and V
OUT
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 68. The measured V
OUT
and efficiency characteristics for the
regulated and unregulated outputs are shown in Figure 37 to Figure 36.
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, V
DO
, at the specified load current should be as low as possible to maintain
efficiency. For a low-cost 150 mA LDO, a V
DO
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:
V
I-min
= V
DO-max
+ V
O-max
.
This means in order to determine V
I
for worst-case condition, the user must take the maximum values for V
DO
and V
O
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 V
I-
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 V
I-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
R
DS-on
negligible and allowing the entire converter input voltage to drop across the primary. At this point the
secondary reaches its maximum voltage of
V
S-max
= V
IN-max
× n
with V
IN-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 V
S-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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