ZHCSB64A June 2013 – September 2014 SN6501-Q1
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The SN6501-Q1 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 an asynchronous divider that provides two complementary output signals, S and S, with a 50% duty cycle. A subsequent break-before-make logic inserts a dead-time between the high-pulses of the two signals. The resulting output signals, G1 and G2, present the gate-drive signals for the output transistors Q1 and Q2. As shown in Figure 40, before either one of the gates can assume logic high, there must be a short time period during which both signals are low and both transistors are high-impedance. This short period, known as break-before-make time, is required to avoid shorting out both ends of the primary.
For this design example, use the parameters listed in Table 1 as design parameters.
|DESIGN PARAMETER||EXAMPLE VALUE|
|Input voltage range||3.3 V ± 3%|
|Output voltage||5 V|
|Maximum load current||100 mA|
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 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 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.
The minimum requirements for a suitable low dropout regulator are:
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.
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 2 lists the maximum secondary voltages for various turns ratios commonly applied in push-pull converters with 100 mA output drive.
|CONFIGURATION||VIN-max [V]||TURNS-RATIO||VS-max [V]||VI-max [V]|
|3.3 VIN to 3.3 VOUT||3.6||1.5 ± 3%||5.6||6 to 10|
|3.3 VIN to 5 VOUT||3.6||2.2 ± 3%||8.2||10|
|5 VIN to 5 VOUT||5.5||1.5 ± 3%||8.5||10|
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.
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 VIN, 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.
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:
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.
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:
VS-min must be large enough to allow for a maximum voltage drop, VF-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 VF-max gives the minimum secondary voltage with:
Then calculating the available minimum primary voltage, VP-min, involves subtracting the maximum possible drain-source voltage of the SN6501, VDS-max, from the minimum converter input voltage VIN-min:
VDS-max however, is the product of the maximum RDS(on) and ID values for a given supply specified in the SN6501 data sheet:
For a 3.3 VIN to 5 VOUT 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 VF-max = 0.2 V,
VDO-max = 0.2 V, and VO-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 VIN-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 RDS-max = 3 Ω and ID-max = 150 mA.
Inserting the values above into Equation 10 yields a minimum turns ratio of:
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%.
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.
|V x T
|1:1.1 ±2%||7||2500||6.73 x 10.05 x 4.19||3.3 V → 3.3 V||No||Figure 1
|760390011||Wurth Electronics/ Midcom|
|1:1.1 ±2%||11||5 V → 5 V||Figure 3
|1:1.7 ±2%||3.3 V → 5 V||Figure 5
|1:1.3 ±2%||3.3 V → 3.3 V
5 V → 5 V
|1:2.1 ±2%||3.3 V → 5 V||Figure 11
|1.23:1 ±2%||5 V → 3.3 V||Figure 13
|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
|1:1.7 ±2%||3.3 V → 5 V||Figure 19
|1:1.3 ±2%||3.3 V → 3.3 V
5 V → 5 V
|1:2.1 ±2%||3.3 V → 5 V||Figure 25
|1.3:1 ±2%||5 V → 3.3 V||Figure 27
See Table 3 for application curves.
The SN6501 can drive push-pull converters that provide high output voltages of up to 30 V, or bipolar outputs of up to ±15 V. Using commercially available center-tapped transformers, with their rather low turns ratios of 0.8 to 5, requires different rectifier topologies to achieve high output voltages. Figure 46 to Figure 49 show some of these topologies together with their respective open-circuit output voltages.
The following application circuits are shown for a 3.3 V input supply commonly taken from the local, regulated micro-controller supply. For 5 V input voltages requiring different turn ratios refer to the transformer manufacturers and their websites listed in Table 4.
|Murata Power Solutions||http://www.murata-ps.com|
|Wurth Electronics Midcom Inc||http://www.midcom-inc.com|
Certain components might not possess AEC-Q100 Q1 qualification. For more detailed information on qualified components for automotive applications please refer to the automotive web page: http://www.ti.com/lsds/ti/apps/automotive/applications.page.