Datasheet

LM25085, LM25085-Q1
SNVS593H –OCTOBER 2008–REVISED MARCH 2013 www.ti.com
If the PFET R
DS(ON)
is used for current limit detection, note that it typically has a positive temperature coefficient.
At 100°C the R
DS(ON)
may be as much as 50% higher than the value at 25°C which could result in incorrect
current limiting if not accounted for when determining the value of the R
ADJ
resistor. The PFET Total Gate
Charge determines most of the power dissipation in the LM25085 due to the repetitive charge and discharge of
the PFET’s gate capacitance by the gate driver (powered from the VCC regulator). The LM25085’s internal
power dissipation can be calculated from the following:
P
DISS
= V
IN
x ((Q
G
x F
S
) + I
IN
)
where
• Q
G
is the PFET Total Gate Charge obtained from its datasheet
• F
S
is the switching frequency
• I
IN
is the LM25085's operating current obtained from Figure 5 (12)
Using the Thermal Resistance specifications in 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 LM25085 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
1.25V, causing V
OUT
to ramp up in a proportional manner. The soft-start ramp time is typically 2.5ms.
In addition to controlling the initial power up cycle, the soft-start circuit also activates when the LM25085 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
V
OUT
, the internal reference voltage provided by the soft-start circuit to the regulation comparator is reduced
along with FB. When the over-current or short circuit condition is removed, V
OUT
returns to the regulated value at
a rate determined by the soft-start ramp. This feature helps prevent the output voltage from overshooting
following an overload event.
THERMAL SHUTDOWN
The LM25085 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.
Applications Information
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with the following design example. Referring
to Block Diagram, the circuit is to be configured for the following specifications:
• V
OUT
= 5V
• V
IN
= 7V to 42V, 12V Nominal
• Maximum load current (I
OUT(max)
) = 5A
• Minimum load current (I
OUT(min)
) = 600mA (for continuous conduction mode)
• Switching Frequency (F
SW
) = 300kHz
• Maximum allowable output ripple (V
OS
) = 5mVp-p
• Selected PFET: Vishay Si7465
• R
FB1
and R
FB2
: These resistors set the output voltage. The ratio of these resistors is calculated from:
R
FB2
/R
FB1
= (V
OUT
/1.25V) - 1
For this example, R
FB2
/ R
FB1
= 3. Typically, R
FB1
and R
FB2
should be chosen from standard value resistors in
the range of 1kΩ to 20kΩ which satisfy the above ratio. For this example, R
FB2
= 10kΩ, and R
FB1
= 3.4kΩ.
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