SLUU130A – September 2002 – Revised February 2003

4.4Output Capacitor Selection

Selection of the output capacitor is based on many application variables, including function, cost, size, and availability. The minimum allowable output capacitance is determined by the amount of inductor ripple current and the allowable output ripple in equation (3).

COUT(min)

+

 

IRIPPLE

 

+

1.25 A

 

 

+ 22 mF

(3)

8 f VRIPPLE

8 600 kHz

12 mV

 

 

 

 

 

 

In this design, COUT(min) is 22 ∝ F with VRIPPLE = 12 mV to allow for some margin. However, this only affects the capacitive component of the ripple voltage. In addition, the voltage component due to the capacitor ESR must

be considered in equation (4).

ESRCout v

VRIPPLE

+

0.012 V

+ 9.6 mW

(4)

IRIPPLE

 

1.25 A

 

 

 

 

 

For compactness while maintaining transient response capability, two 22-∝F ceramic capacitors are fitted in parallel. The total ESR of these capacitors is below 3 mΩ , and contributes only a few mV to the output voltage ripple.

4.5MOSFET Selection

The small physical size of this design requires the use of a single SO-8 package which contains dual N-channel

MOSFETs. MOSFETs with an RDS(on) of 18 mΩ are selected to keep the conduction losses to a manageable amount at full load.

4.6Short Circuit Protection

The TPS40003 implements short circuit protection by comparing the voltage across the topside MOSFET while

it is on to a voltage dropped from VDD by RLIM due to an internal current source of 15 ∝ A inside pin 1. Due to tolerances in the current source and variations in the power MOSFET on-voltage versus temperature, the short

circuit level can protect against gross overcurrent conditions only, and should be set higher than rated load. In this particular case, RLIM is selected as:

2.5

IOUT 0.018 W

 

(5)

RLIM + R1 +

 

 

+ 15 kW

 

 

15 mA

 

For this design, RLIM = 15 kΩ , and the factor of 2.5 in the equation accounts for the variations in component tolerances over temperature and output current ripple. The high currents that are switched under short circuit

conditions may cause SW pin 8 to be driven below ground several volts, possibly injecting substrate current which can cause improper operation of the device. A 3.3-Ωresistor has been placed in series with this pin to limit its excursion to safe levels.

TPS40003-Based 5-A Converter in Less Than One Square Inch

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Texas Instruments TPS40003 manual Output Capacitor Selection, Mosfet Selection, Short Circuit Protection
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TPS40003 specifications

The Texas Instruments TPS40003 is a versatile, high-performance synchronous buck controller designed to efficiently regulate output voltage in a range of applications. This device is particularly suited for power supply designs where space and efficiency are critical, such as telecommunications, industrial automation, and consumer electronics.

One of the standout features of the TPS40003 is its ability to operate over a wide input voltage range of 4.5V to 36V. This flexibility allows it to be effectively utilized in various scenarios, accommodating different power supply systems and ensuring compatibility with existing infrastructure. Furthermore, the output voltage can be adjusted down to as low as 0.8V, making it suitable for powering low-voltage microcontrollers and FPGAs.

The TPS40003 incorporates advanced voltage mode control, which enhances transient response and stability. This control method allows for quick adjustments in duty cycle in response to load changes, ensuring a steady output voltage even under varying loads. Coupled with a programmable soft-start feature, the controller minimizes inrush current during startup, protecting sensitive downstream components.

Another significant advantage of the TPS40003 is its integration of a high-side and low-side MOSFET driver. This synchronous operation significantly improves power efficiency by reducing conduction losses. The device operates with a high switching frequency, typically around 200 kHz, allowing for the use of smaller passive components and ultimately leading to a more compact design.

Thermal performance is another essential characteristic of the TPS40003. With built-in thermal shutdown and overcurrent protection features, it promotes reliable operation under various thermal conditions, enhancing the overall robustness of the power supply design. Additionally, the device's wide range of compensation options allows designers to fine-tune performance according to specific application requirements.

In terms of packaging, the TPS40003 is offered in a compact, thermally enhanced HTQFN package, which helps in conserving PCB space while maintaining efficient heat dissipation.

Overall, the Texas Instruments TPS40003 controller stands out as a robust, feature-rich solution for DC-DC conversion applications. With its wide operating voltage range, synchronous operation, and protective features, it provides designers with a reliable, efficient means of managing power in today's demanding electronic systems.
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