LM25085A

SNVS601B – JANUARY 2009 – REVISED MARCH 2013

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P-CHANNEL MOSFET SELECTION

The PFET must be rated for the maximum input voltage, with some margin above that to allow for transients and

provided to the PFET is 7.7 volts for VIN greater than 8.5V. However, if the circuit is to be operated at lower

input operating voltage for the LM25085A is 4.5V.

Similar to NFETs, the case or exposed thermal pad for a PFET is electrically connected to the drain terminal.

When designing a PFET buck regulator the drain terminal is connected to the switching node. This situation

requires a trade-off between thermal and EMI performance since increasing the PC board area of the switching

node to aid the PFET power dissipation also increases radiated noise, possibly disrupting the circuit operation.

Typically the switching node area is kept to a reasonable minimum and the PFET peak current is derated to stay

A PFET with a higher gate charge has a corresponding slower switching speed, leading to higher switching

losses and affecting the PFET power dissipation.

Charge determines most of the power dissipation in the LM25085A due to the repetitive charge and discharge of

the PFET’s gate capacitance by the gate driver (powered from the VCC regulator). The LM25085A’s internal

power dissipation can be calculated from the following:

(12)

the LM25085A's operating current obtained from Figure 5. Using the Thermal Resistance specifications in the

Electrical Characteristics, the approximate junction temperature can be determined. If the calculated junction

temperature is near the maximum operating temperature of 125°C, either the switching frequency must be

reduced, or a PFET with a smaller Total Gate Charge must be used.

SOFT-START

The internal soft-start feature of the LM25085A allows the regulator to gradually reach a steady state operating

point at power up, thereby reducing startup stresses and current surges. Upon turn-on, when Vcc reaches its

under-voltage lockout threshold, the internal soft-start circuit ramps the feedback reference voltage from 0V to

In addition to controlling the initial power up cycle, the soft-start circuit also activates when the LM25085A is

enabled by releasing the RT pin, and when the circuit is shutdown and restarted by the internal Thermal

Shutdown circuit.

If the voltage at FB is below the regulation threshold value due to an over-current condition or a short circuit at

Vout, the internal reference voltage provided by the soft-start circuit to the regulation comparator is reduced

a rate determined by the soft-start ramp. This feature helps prevent the output voltage from over-shooting

following an overload event.

THERMAL SHUTDOWN

The LM25085A should be operated such that the junction temperature does not exceed 125°C. If the junction

temperature increases above that, an internal Thermal Shutdown circuit activates at 170°C (typical) to disable

the VCC regulator and the gate driver, and discharge the soft-start capacitor. This feature helps prevent

catastrophic failures from accidental device overheating. When the junction temperature falls below 150°C

(typical hysteresis = 20°C), the gate driver is enabled, the soft-start circuit is released, and normal operation

resumes.

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