Table 1. Design Parameters

9.2.2.1 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.

9.2.2.2 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, 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:
Equation 1. 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
Equation 2. 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 2 lists the maximum secondary voltages for various turns ratios commonly applied in push-pull converters with 100 mA output drive.

9.2.2.3 Diode Selection

A rectifier diode should always possess low-forward voltage to provide as much voltage to the converter output as possible. When used in high-frequency switching applications, such as the SN6501 however, the diode must also possess a short recovery time. Schottky diodes meet both requirements and are therefore strongly recommended in push-pull converter designs. A good choice for low-volt applications and ambient temperatures of up to 85°C is the low-cost Schottky rectifier MBR0520L with a typical forward voltage of 275 mV at 100-mA forward current. For higher output voltages such as ±10 V and above use the MBR0530 which provides a higher DC blocking voltage of 30 V.

Lab measurements have shown that at temperatures higher than 100°C the leakage currents of the above Schottky diodes increase significantly. This can cause thermal runaway leading to the collapse of the rectifier output voltage. Therefore, for ambient temperatures higher than 85°C use low-leakage Schottky diodes, such as RB168M-40.

Figure 42. Diode Forward Characteristics for MBR0520L (Left) and MBR0530 (Right)

9.2.2.4 Capacitor Selection

The capacitors in the converter circuit in Figure 45 are multi-layer ceramic chip (MLCC) capacitors.

As with all high speed CMOS ICs, the SN6501 requires a bypass capacitor in the range of 10 nF to 100 nF.

The input bulk capacitor at the center-tap of the primary supports large currents into the primary during the fast switching transients. For minimum ripple make this capacitor 1 μF to 10 μF. In a 2-layer PCB design with a dedicated ground plane, place this capacitor close to the primary center-tap to minimize trace inductance. In a 4-layer board design with low-inductance reference planes for ground and V_IN, the capacitor can be placed at the supply entrance of the board. To ensure low-inductance paths use two vias in parallel for each connection to a reference plane or to the primary center-tap.

The bulk capacitor at the rectifier output smoothes the output voltage. Make this capacitor 1 μF to 10 μF.

The small capacitor at the regulator input is not necessarily required. However, good analog design practice suggests, using a small value of 47 nF to 100 nF improves the regulator’s transient response and noise rejection.

The LDO output capacitor buffers the regulated output for the subsequent isolator and transceiver circuitry. The choice of output capacitor depends on the LDO stability requirements specified in the data sheet. However, in most cases, a low-ESR ceramic capacitor in the range of 4.7 μF to 10 μF will satisfy these requirements.

9.2.2.5 Transformer Selection

9.2.2.5.1 V-t Product Calculation

To prevent a transformer from saturation its V-t product must be greater than the maximum V-t product applied by the SN6501. The maximum voltage delivered by the SN6501 is the nominal converter input plus 10%. The maximum time this voltage is applied to the primary is half the period of the lowest frequency at the specified input voltage. Therefore, the transformer’s minimum V-t product is determined through:

Equation 3.

Inserting the numeric values from the data sheet into the equation above yields the minimum V-t products of

Equation 4.

Common V-t values for low-power center-tapped transformers range from 22 Vμs to 150 Vμs with typical footprints of 10 mm x 12 mm. However, transformers specifically designed for PCMCIA applications provide as little as 11 Vμs and come with a significantly reduced footprint of 6 mm x 6 mm only.

While Vt-wise all of these transformers can be driven by the SN6501, other important factors such as isolation voltage, transformer wattage, and turns ratio must be considered before making the final decision.

9.2.2.5.2 Turns Ratio Estimate

Assume the rectifier diodes and linear regulator has been selected. Also, it has been determined that the transformer choosen must have a V-t product of at least 11 Vμs. However, before searching the manufacturer websites for a suitable transformer, the user still needs to know its minimum turns ratio that allows the push-pull converter to operate flawlessly over the specified current and temperature range. This minimum transformation ratio is expressed through the ratio of minimum secondary to minimum primary voltage multiplied by a correction factor that takes the transformer’s typical efficiency of 97% into account:

Equation 5. V_P-min = V_IN-min - V_DS-max

V_S-min must be large enough to allow for a maximum voltage drop, V_F-max, across the rectifier diode and still provide sufficient input voltage for the regulator to remain in regulation. From the LDO SELECTION section, this minimum input voltage is known and by adding V_F-max gives the minimum secondary voltage with:

Equation 6. V_S-min = V_F-max + V_DO-max + V_O-max

Figure 43. Establishing the Required Minimum Turns Ratio Through N_min = 1.031 × V_S-min / V_P-min

Then calculating the available minimum primary voltage, V_P-min, involves subtracting the maximum possible drain-source voltage of the SN6501, V_DS-max, from the minimum converter input voltage V_IN-min:

Equation 7. V_P-min = V_IN-min – V_DS-max

V_DS-max however, is the product of the maximum R_DS(on) and I_D values for a given supply specified in the SN6501 data sheet:

Equation 8. V_DS-max = R_DS-max × I_Dmax

Then inserting Equation 8 into Equation 7 yields:

Equation 9. V_P-min = V_IN-min - R_DS-max x I_Dmax

and inserting Equation 9 and Equation 6 into Equation 5 provides the minimum turns ration with:

Equation 10.

Example:

For a 3.3 V_IN to 5 V_OUT converter using the rectifier diode MBR0520L and the 5 V LDO TPS76350, the data sheet values taken for a load current of 100 mA and a maximum temperature of 85°C are V_F-max = 0.2 V,
V_DO-max = 0.2 V, and V_O-max = 5.175 V.

Then assuming that the converter input voltage is taken from a 3.3 V controller supply with a maximum ±2% accuracy makes V_IN-min = 3.234 V. Finally the maximum values for drain-source resistance and drain current at 3.3 V are taken from the SN6501 data sheet with R_DS-max = 3 Ω and I_D-max = 150 mA.

Inserting the values above into Equation 10 yields a minimum turns ratio of:

Equation 11.

Most commercially available transformers for 3-to-5 V push-pull converters offer turns ratios between 2.0 and 2.3 with a common tolerance of ±3%.

9.2.2.5.3 Recommended Transformers

Depending on the application, use the minimum configuration in Figure 44 or standard configuration in Figure 45.

Figure 44. Unregulated Output for Low-Current Loads With Wide Supply Range

Figure 45. Regulated Output for Stable Supplies and High Current Loads

The Wurth Electronics Midcom isolation transformers in Table 3 are optimized designs for the SN6501, providing high efficiency and small form factor at low-cost.

The 1:1.1 and 1:1.7 turns-ratios are designed for logic applications with wide supply rails and low load currents. These applications operate without LDO, thus achieving further cost-reduction.

Table 3. Recommended Isolation Transformers Optimized for SN6501

Turns Ratio	V x T (Vμs)	Isolation (VRMS)	Dimensions (mm)	Application	LDO	Figures	Order No.	Manufacturer
1:1.1 ±2%	7	2500	6.73 x 10.05 x 4.19	3.3 V → 3.3 V	No	Figure 1 Figure 2	760390011	Wurth Electronics/ Midcom
1:1.1 ±2%	11			5 V → 5 V		Figure 3 Figure 4	760390012
1:1.7 ±2%				3.3 V → 5 V		Figure 5 Figure 6	760390013
1:1.3 ±2%				3.3 V → 3.3 V 5 V → 5 V	Yes	Figure 7 Figure 8 Figure 9 Figure 10	760390014
1:2.1 ±2%				3.3 V → 5 V		Figure 11 Figure 12	760390015
1.23:1 ±2%				5 V → 3.3 V		Figure 13 Figure 14	750313710
1:1.1 ±2%	11	5000	9.14 x 12.7 x 7.37	3.3 V → 3.3 V	No	Figure 15Figure 16	750313734
1:1.1 ±2%				5 V → 5 V		Figure 17 Figure 18	750313734
1:1.7 ±2%				3.3 V → 5 V		Figure 19 Figure 20	750313769
1:1.3 ±2%				3.3 V → 3.3 V 5 V → 5 V	Yes	Figure 21 Figure 22 Figure 23 Figure 24	750313638
1:2.1 ±2%				3.3 V → 5 V		Figure 25 Figure 26	750313626
1.3:1 ±2%				5 V → 3.3 V		Figure 27 Figure 28	750313638

Table 4. Transformer Manufacturers