Datasheet

Duty Cycle =
V
OUT
+ V
DIODE
- V
IN
V
OUT
+ V
DIODE
- V
SW
LMR62014
www.ti.com SNVS735B –OCTOBER 2011–REVISED APRIL 2013
SETTING THE OUTPUT VOLTAGE
The output voltage is set using the external resistors R1 and R2 (see Basic Application Circuit). A value of
approximately 13.3 kΩ is recommended for R2 to establish a divider current of approximately 92 µA. R1 is
calculated using the formula:
R1 = R2 X (V
OUT
/1.23 − 1) (2)
Figure 19. Basic Application Circuit
DUTY CYCLE
The maximum duty cycle of the switching regulator determines the maximum boost ratio of output-to-input
voltage that the converter can attain in continuous mode of operation. The duty cycle for a given boost
application is defined as:
(3)
This applies for continuous mode operation.
INDUCTANCE VALUE
The first question we are usually asked is: “How small can I make the inductor?” (because they are the largest
sized component and usually the most costly). The answer is not simple and involves trade-offs in performance.
Larger inductors mean less inductor ripple current, which typically means less output voltage ripple (for a given
size of output capacitor). Larger inductors also mean more load power can be delivered because the energy
stored during each switching cycle is:
E = L/2 X (lp)
2
where
• “lp” is the peak inductor current. (4)
An important point to observe is that the LMR62014 will limit its switch current based on peak current. This
means that since lp(max) is fixed, increasing L will increase the maximum amount of power available to the load.
Conversely, using too little inductance may limit the amount of load current which can be drawn from the output.
Best performance is usually obtained when the converter is operated in “continuous” mode at the load current
range of interest, typically giving better load regulation and less output ripple. Continuous operation is defined as
not allowing the inductor current to drop to zero during the cycle. It should be noted that all boost converters shift
over to discontinuous operation as the output load is reduced far enough, but a larger inductor stays “continuous”
over a wider load current range.
Copyright © 2011–2013, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LMR62014