7.3.1 Peak Current Control

The internal switch turns on until the inductor current reaches the typical DC current limit (I_LIM) of 400 mA (TPS61040-Q1) or 250 mA (TPS61041-Q1). Due to the internal propagation delay of typical 100 ns, the actual current exceeds the DC-current limit threshold by a small amount. The typical peak current limit can be calculated:

Equation 1.

where

• V_IN= Input voltage
• L= Selected inductor value
• I_LIM = Typical DC current limit

The higher the input voltage and the lower the inductor value, the greater the peak.

By selecting the TPS6104x-Q1, it is possible to tailor the design to the specific application current limit requirements. A lower current limit supports applications requiring lower output power and allows the use of an inductor with a lower current rating and a smaller form factor. A lower current limit usually has a lower output-voltage ripple as well.

7.3.2 Soft Start

All inductive step-up converters exhibit high inrush current during start-up if no special precaution is made. This can cause voltage drops at the input rail during start-up and may result in an unwanted or early system shutdown.

The TPS6104x-Q1 limits this inrush current by increasing the current limit in two steps starting from for 256 cycles to for the next 256 cycles, and then full current limit (see Figure 15).

7.3.3 Enable

Pulling the enable (EN) to ground shuts down the device reducing the shutdown current to 1 µA (typical). Because there is a conductive path from the input to the output through the inductor and Schottky diode, the output voltage is equal to the input voltage during shutdown. The enable pin must be terminated and must not be left floating. Using a small external transistor disconnects the input from the output during shutdown as shown in Figure 17.

7.3.4 Undervoltage Lockout

An undervoltage lockout prevents misoperation of the device at input voltages below typical 1.5 V. When the input voltage is below the undervoltage threshold the main switch is turned off.

7.3.5 Thermal Shutdown

An internal thermal shutdown is implemented and turns off the internal MOSFETs when the typical junction temperature of 168°C is exceeded. The thermal shutdown has a hysteresis of typically 25°C. This data is based on statistical means and is not tested during the regular mass production of the IC.