Datasheet

LM2650
www.ti.com SNVS133C –JUNE 1999–REVISED APRIL 2013
Note that as the load current decreases, it takes increasingly longer periods for the load current to discharge
C
OUT
through the hysteretic window, and as the load current increases, the periods become even shorter. It can
be seen from the above observation that the switching frequency of the hysteretic loop varies as the load varies.
The switching frequency can be approximated using
(6)
Here f is the switching frequency in hertz, I is the load current in amperes, C
OUT
is the value of the capacitor in
farads, and V
OUT_PP
is the magnitude of the AC ripple voltage in volts. Typical switching frequencies range
anywhere from a few hertz for very light loads to a few thousand hertz for light loads bordering on the moderate
level.
Application Circuits
Figure 18 is a schematic of the typical application circuit. use the component values shown in the figure and
those contained in Table 1 to build a 5V, 3A, or 3.3V, 3A step-down DC/DC converter. As with the design of any
DC/DC converter, the design of these circuits involved tradeoffs between efficiency, size, and cost. Here more
weight was given to efficiency than to size as evidenced by the low switching frequency which keeps switching
losses low but pushes the value and size of the inductor up.
From a smaller circuit, use the component values shown in Figure 18 and those contained in Table 3. These
circuits trade slightly higher switching losses for a much smaller inductor. Note, Figure 18 does not show R
FA
, the
resistor required to adjust the switching frequency from 90 kHz up to 200 kHz. Connect R
FA
between the FREQ
ADJ pin and ground.
Figure 18. The Typical 90 kHz Application Circuit
Table 1. Components for the Typical 90 kHz Application Circuit
Input Voltage 7V to 18V IN
Applicable Cell Stacks 8 to 12 Cell NiCd or NiMh, 3 to 4 Cell Li Ion, 8 to 11 Cell Alkaline, 6 Cell Lead Acid
Output 5V, 3A Out 3.3V, 3A out
Input Capacitor C
IN
2 x 22 μF, 35V AVX TPS 2 x 22 μF, 35V AVX TPS
Series or Sprague 593D Series Series or Sprague 593D Series
Inductor L1 40μH (See Table 2) 33μH (See Table 2)
Output Capacitor C
OUT
3x220 μF, 10V AVX TPS 3x220 μF, 10V AVX TPS
Series or Sprague 593D Series Series or Sprague 593D Series
Feedback Resistors R1 and R2 R1 = 75kΩ, 1%, R2 = 24.9kΩ, 1%, R1 = 41.2kΩ, 1%, R2 = 24.9kΩ, 1%,
Compensation Components R
C
, C
C
, R
C
= 37.4 kΩ, C
C
= 4.7 nF, R
C
= 23.2 kΩ, C
C
= 8.2 nF,
R
3
, and C
10
R
3
= 3.57 kΩ, C
10
= 5.6 nF R
3
= 2.0 kΩ, C
10
= 10 nF
Sleep Resistors R
SIA
and R
SOA
R
SIA
= 33 kΩ, R
SOA
= 200 kΩ R
SIA
= 39 kΩ, R
SOA
= 130 kΩ
Copyright © 1999–2013, Texas Instruments Incorporated Submit Documentation Feedback 11
Product Folder Links: LM2650