Datasheet

'i
L
=
(in Amps)
V
IN
D
2Lfs
V
IN
R
DSON
0.144 fs
L >
( )
D
D'
2
-1
( )
D
D'
+1
(in H)
Hz
1
2S(R
C
+ R
O
)C
C
f
PC
=
Hz
1
f
ZC
=
2SR
C
C
C
LM2622
www.ti.com SNVS068E –MAY 2000–REVISED MARCH 2013
(3)
where
• R
O
is the output impedance of the error amplifier, approximately 1MegΩ (4)
For most applications, performance can be optimized by choosing values within the range 5kΩ ≤ R
C
≤ 20kΩ (R
C
can be up to 200kΩ if C
C2
is used, see HIGH OUTPUT CAPACITOR ESR COMPENSATION) and 680pF ≤ C
C
≤
4.7nF. Refer to the Application Information section for recommended values for specific circuits and conditions.
Refer to the COMPENSATION section for other design requirement.
COMPENSATION
This section will present a general design procedure to help insure a stable and operational circuit. The designs
in this datasheet are optimized for particular requirements. If different conversions are required, some of the
components may need to be changed to ensure stability. Below is a set of general guidelines in designing a
stable circuit for continuous conduction operation (loads greater than approximately 75mA), in most all cases this
will provide for stability during discontinuous operation as well. The power components and their effects will be
determined first, then the compensation components will be chosen to produce stability.
INDUCTOR AND DIODE SELECTION
Although the inductor sizes mentioned earlier are fine for most applications, a more exact value can be
calculated. To ensure stability at duty cycles above 50%, the inductor must have some minimum value
determined by the minimum input voltage and the maximum output voltage. This equation is:
where
• fs is the switching frequency
• D is the duty cycle
• R
DSON
is the ON resistance of the internal switch taken from the graph "R
DSON
vs. V
IN
" in the Typical
Performance Characteristics section (5)
This equation is only good for duty cycles greater than 50% (D>0.5), for duty cycles less than 50% the
recommended values may be used. The corresponding inductor current ripple as shown in Figure 18 (a) is given
by:
(6)
The inductor ripple current is important for a few reasons. One reason is because the peak switch current will be
the average inductor current (input current or I
LOAD
/D') plus Δi
L
. As a side note, discontinuous operation occurs
when the inductor current falls to zero during a switching cycle, or Δi
L
is greater than the average inductor
current. Therefore, continuous conduction mode occurs when Δi
L
is less than the average inductor current. Care
must be taken to make sure that the switch will not reach its current limit during normal operation. The inductor
must also be sized accordingly. It should have a saturation current rating higher than the peak inductor current
expected. The output voltage ripple is also affected by the total ripple current.
The output diode for a boost regulator must be chosen correctly depending on the output voltage and the output
current. The typical current waveform for the diode in continuous conduction mode is shown in Figure 18 (b). The
diode must be rated for a reverse voltage equal to or greater than the output voltage used. The average current
rating must be greater than the maximum load current expected, and the peak current rating must be greater
than the peak inductor current. During short circuit testing, or if short circuit conditions are possible in the
application, the diode current rating must exceed the switch current limit. Using Schottky diodes with lower
forward voltage drop will decrease power dissipation and increase efficiency.
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