Datasheet
BST
SW
VCC
IS
GND
LM5575
C
OUT
D1
L1
D2
V
OUT
BST
SW
VCC
IS
GND
LM5575
V
OUT
D2
D1
L1
C
OUT
LM5575
SNVS471G –JANUARY 2007–REVISED APRIL 2013
www.ti.com
If a network analyzer is available, the modulator gain can be measured and the error amplifier gain can be
configured for the desired loop transfer function. If a network analyzer is not available, the error amplifier
compensation components can be designed with the guidelines given. Step load transient tests can be
performed to verify acceptable performance. The step load goal is minimum overshoot with a damped response.
C6 can be added to the compensation network to decrease noise susceptibility of the error amplifier. The value
of C6 must be sufficiently small since the addition of this capacitor adds a pole in the error amplifier transfer
function. This pole must be well beyond the loop crossover frequency. A good approximation of the location of
the pole added by C6 is: f
p2
= fz x C5 / C6.
BIAS POWER DISSIPATION REDUCTION
Buck regulators operating with high input voltage can dissipate an appreciable amount of power for the bias of
the IC. The V
CC
regulator must step-down the input voltage V
IN
to a nominal V
CC
level of 7V. The large voltage
drop across the V
CC
regulator translates into a large power dissipation within the Vcc regulator. There are several
techniques that can significantly reduce this bias regulator power dissipation. Figure 20 and Figure 21 depict two
methods to bias the IC from the output voltage. In each case the internal Vcc regulator is used to initially bias the
VCC pin. After the output voltage is established, the VCC pin potential is raised above the nominal 7V regulation
level, which effectively disables the internal V
CC
regulator. The voltage applied to the VCC pin should never
exceed 14V. The V
CC
voltage should never be larger than the V
IN
voltage.
Figure 20. VCC Bias from VOUT for 8V < VOUT < 14V
Figure 21. VCC Bias with Additional Winding on the Output Inductor
18 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM5575