RON =

5V x (8V ± 1.5V)

8V x 250 kHz x 1.14 x 10

-10

- 1.4 k: = 141 k:

LM2694

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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 VCC 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 VCC 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:

•

VOUT = 5V

•

VIN = 8V to 30V

•

FS = 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 = (VOUT/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 k

Ω - 10

k

Ω. A value of 2.5 kΩ will be used for R1 and for R2.

RON, FS: RON can be chosen using Equation 6 to set the nominal frequency, or from Equation 5 if the on-time at

a particular VIN 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 VIN of 8V,

(10)

A value of 140 k

Ω will be used for RON, 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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