Datasheet

R
ON
=
5V x (8V ± 1.5V)
8V x 250 kHz x 1.14 x 10
-10
- 1.4 k: = 141 k:
LM2694
www.ti.com SNVS444A –MAY 2006–REVISED APRIL 2013
Softstart
The softstart feature allows the converter to gradually reach a steady state operating point, thereby reducing
start-up stresses and current surges. Upon turn-on, after V
CC
reaches the under-voltage threshold, an internal 12
µA current source charges up the external capacitor at the SS pin to 2.5V. The ramping voltage at SS (and the
non-inverting input of the regulation comparator) ramps up the output voltage in a controlled manner.
An internal switch grounds the SS pin if V
CC
is below the under-voltage lockout threshold, if a thermal shutdown
occurs, or if the RON/SD pin is grounded.
Thermal Shutdown
The LM2694 should be operated so the junction temperature does not exceed 125°C. If the junction temperature
increases, an internal Thermal Shutdown circuit, which activates (typically) at 175°C, takes the controller to a low
power reset state by disabling the buck switch and the on-timer, and grounding the Softstart pin. This feature
helps prevent catastrophic failures from accidental device overheating. When the junction temperature reduces
below 155°C (typical hysteresis = 20°C), the Softstart pin is released and normal operation resumes.
APPLICATIONS INFORMATION
EXTERNAL COMPONENTS
The procedure for calculating the external components is illustrated with a design example. Referring to the
Block Diagram, the circuit is to be configured for the following specifications:
• V
OUT
= 5V
• V
IN
= 8V to 30V
• F
S
= 250 kHz
• Minimum load current = 100 mA
• Maximum load current = 600 mA
• Softstart time = 5 ms.
R1 and R2: These resistors set the output voltage, and their ratio is calculated from:
R1/R2 = (V
OUT
/2.5V) - 1 (9)
R1/R2 calculates to 1.0. The resistors should be chosen from standard value resistors in the range of 1.0 kΩ - 10
kΩ. A value of 2.5 kΩ will be used for R1 and for R2.
R
ON
, F
S
: R
ON
can be chosen using Equation 6 to set the nominal frequency, or from Equation 5 if the on-time at
a particular V
IN
is important. A higher frequency generally means a smaller inductor and capacitors (value, size
and cost), but higher switching losses. A lower frequency means a higher efficiency, but with larger components.
Generally, if PC board space is tight, a higher frequency is better. The resulting on-time and frequency have a
±25% tolerance. Using Equation 6 at a V
IN
of 8V,
(10)
A value of 140 kΩ will be used for R
ON
, yielding a nominal frequency of 252 kHz.
L1: The guideline for choosing the inductor value in this example is that it must keep the circuit’s operation in
continuous conduction mode at minimum load current. This is not a strict requirement since the LM2694
regulates correctly when in discontinuous conduction mode, although at a lower frequency. However, to provide
an initial value for L1 the above guideline will be used.
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