Datasheet

Duty Cycle =
V
OUT
+ V
DIODE
- V
IN
V
OUT
+ V
DIODE
- V
SW
LMR64010
SNVS736B –SEPTEMBER 2011–REVISED APRIL 2013 www.ti.com
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 17. 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.
The equation shown for calculating duty cycle incorporates terms for the FET switch voltage and diode forward
voltage. The actual duty cycle measured in operation will also be affected slightly by other power losses in the
circuit such as wire losses in the inductor, switching losses, and capacitor ripple current losses from self-heating.
Therefore, the actual (effective) duty cycle measured may be slightly higher than calculated to compensate for
these power losses. A good approximation for effctive duty cycle is :
DC (eff) = (1 - Efficiency x (V
IN
/V
OUT
))
where
• the efficiency can be approximated from the curves provided. (4)
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 tradeoffs 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. (5)
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