feature

High power switch mode DC-DC controller can provide more than 13A output voltage can be adjusted from 1.2V to 3.6V 85% efficiency cumulative accuracy < 3% over line, load and temperature changes Over voltage and short circuit protection built-in soft start no over tune

application

High-efficiency power supply for I/O and AGP power ASICs for desktop computers High-efficiency power supplies for DSPs Adjustable buck power supplies

illustrate

The FAN5037 is a high power switching DC-DC controller that provides high efficiency power supply for all low voltage applications. This controller has a built-in soft-start function by reducing magnetizing inrush current and output overshoot. When combined with the appropriate external circuitry, the FAN5037 can deliver load currents up to 13A with efficiencies as high as 88%. The FAN5037 can generate an output voltage of 3.6V from 1.2V to 1.2V using external resistors. The FAN5037 is designed to operate the control mode under all load conditions with constant turn-on time. An accurate low TC reference eliminates the need for precision external components to achieve the tight tolerance voltage regulation required by many applications. The short-circuit current is provided by the current sensor to provide protection resistors, and overvoltage protection is provided internally.

Application Information: The FAN5037 contains a finely tuned zero TC voltage reference, a duration architecture controller, a high current output driver, low offset error amplifier. The detailed block diagram in this figure shows the FAN5037 working with external components to achieve a high performance switching power supply. Switch Mode Control Loop The main control loop of the switch mode converter includes a current regulator amplifier and a voltage regulator amplifier. The voltage amplifier compares the internal reference of the converter's output voltage divided by an external resistor divider. A current amplifier senses the current obtained by comparing the voltages at the IFBH and IFBL pins, connected to either side of the current sense resistor. The signals from the voltage and current amplifiers are added together, and the result is used to control the off-time oscillator. The current feedback signal also acts as part of the fan 5037 short circuit protection. High Current Output Driver The FAN5037 High Current Output Driver (DRV) contains a high-speed bipolar power transistor in a push-pull configuration.

The output driver is capable of delivering 0.5A in less than 100ns. The drive ground is separate from the entire chip power supply to increase switching noise immunity. Internal Reference The reference in the FAN5037 is a precision bandgap type reference. Its temperature coefficient is adjusted to be close to zero TC. Constant Time Oscillator FAN5037 Switch Mode Oscillator is designed as a fixed switch time oscillator. Time Constant Oscillator A switching comparator that selects between two threshold voltages by a comparator, an external capacitor, and a fixed current source, a variable current source, and an analog current source. An external timing capacitor is alternately enabled and disabled to charge and discharge the fixed current source. The variable current source controls the current and voltage feedback signals from the received error input. The oscillator off time is determined by a variable current source that can be charged from an external capacitor to the high threshold level of the comparator. On time is set by the constant current source of external discharge to reduce the capacitor voltage to the lower limit of the comparator.

The output voltage selection FAN5037 precision reference voltage is adjusted to 1.2V nominal. When using the FAN5037, the system designer has complete flexibility in choosing an output voltage regulator from 1.2V to 3.6V. This is achieved by proper selection of feedback resistors. Probably 0.1% resistor for best output accuracy. The following equation determines the output voltage of the regulator:

The number of input capacitors required to place the capacitors FAN5037 depends on their ripple current rating, which guarantees their rated life. The required quantity can be

Among them, the duty cycle DC=(Vout+Vf, diode)/Vin. For example, outputting 1.5V at 10A, 5V input, and using the rated current of a Sanyo capacitor with 2A ripple specified in Table 1, we have DC=(1.5+.5)/5=0.4,

Short circuit precautions

The FAN5037 uses a current sensing scheme to limit the current when the load output fails. The current sense resistor carries the inductor's peak current, which is greater than the maximum load current caused by the ripple current flowing into the inductor. The fan 5037 will begin to limit the top MOSFET driver threshold voltage (Vth) by reducing the duty cycle of the load when the voltage on the current sense resistor exceeds the shorted comparator. When this happens, the output voltage will temporarily go out of regulation. As the voltage increases through the sense resistor, the duty cycle of the top MOSFET will continue to decrease until the current limit is reached. At this time, the fan 5037 will continue to provide the limiting current voltage level at a lower output. The short-circuit comparator threshold voltage is typically 90 mV with a tolerance of ±10 mV. The ripple current flowing through the inductor in the figure is 0.6 peak. The sense resistor value can be approximated as follows:

TF = tolerance factor of the sense resistor and 0.6A to account for the inductor ripple current. Since the value of the sense resistor is usually less than 10mΩ, the layout of the printed circuit board should be careful. Tracking resistors can lead to serious errors. Find the traces Fan 5037's IFBH and IFBL pins should be Kelvin connected to the pads of the current sense resistor. To minimize the effect of noise, the two traces should be on each other. Schottky Diode In Figure 1, MOSFET Q1 and flyback diode D1 act as complementary switches to maintain a constant current through output inductor L2. Therefore, D1 will be turned off when the power MOSFET is turned off. The power of the diode is a function of the forward voltage of the rated load current when the direct FET is turned off. The following equation can be used to estimate diode power:

Board Design Considerations MOSFET Placement Placement of power MOSFETs is the design of switch-mode regulators. The MOSFET should be placed from the FAN5037 SDRV pin in a way that minimizes the gate drive path length. This trace should be kept below 0.5 inches for best performance. Long lead lengths on this trace can create parasitic inductance and capacitance of the high frequency noise track. Since this voltage can switch nearly 12 volts in about 100 nanoseconds, the resulting ringing and noise will be difficult to suppress. This trace should be routed on only one layer and keep the "quiet" analog pins away from the device: CEXT, IFBH, IFBL, and GND. See Fig. A 4.7Ω resistor input in series with the MOSFET gate reduces the criticality of this layout. See Figure 1. Inductor and Schottky Diode Placement Inductor and flyback Schottky diode need to be placed close to the power supply of the power MOSFET for the same reasons mentioned above. Connect the inductor and the Schottky diode to the forward voltage of the FET and Schottky diode. If possible, it is recommended to convert this node to a plane. This node will be part of the high current path in the design, so it is best to think of it as a parasitic resistance and inductance on a planar node. Since most PC board manufacturers have upper and lower signal layers on the PCB, it is not recommended to use these layers to route high current sections of a regulator design. Because it is more common to use 1 oz. Copper inner layers, those layers recommended for routing high current paths in the design.

Power and Ground Connections The VCCA connection to the 5V power plane should be short and bypass the fan 5037's VCCA pin directly with 0.1µF. The ideal way to connect is through a 5V power strip. A similar arrangement should be made for the VCCP pin connected to +12V. Each ground should have a separate through connection below ground. Bias VCCP with a 12V supply. A 47Ω resistor is used to limit the transient current into VCCP. Capacitor filters above A1 are used to filter the VCCP supply and the transient current capacitance needed to charge the MOSFET gate. This approach provides a sufficiently high gate bias to MOSFET (VGS) voltage, thus reducing the MOSFET's RDS(ON) and its power loss. The diagram provides a gate bias of about 5V, which works well when using typical logic-level MOSFETs. Non-logic level radio data systems (on) should not be used due to higher mosfets.