Datasheet

HB
HO
VCC
CS
CSG
LM5118
VOUT
VOUT
LO
VIN
L1
D1
D2
Cout
Q2
Q1
HS
VBIAS
VCCX
HB
HO
VCC
CS
GND
LM5118
VOUT
LO
CSG
VIN
L1
D1
D2
Cout
Q2
Q1
HS
VCCX
LM5118, LM5118-Q1
SNVS566H APRIL 2008REVISED JANUARY 2014
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Bias Power Dissipation Reduction
Buck or Buck-boost regulators operating with high input voltage can dissipate an appreciable amount of power
while supplying the required bias current of the IC. The VCC regulator must step-down the input voltage VIN to a
nominal VCC level of 7 V. The large voltage drop across the VCC regulator translates into high power dissipation
in the VCC regulator. There are several techniques that can significantly reduce this bias regulator power
dissipation. Figure 26 and Figure 27 depict two methods to bias the IC, one from the output voltage and one from
a separate bias supply. In the first case, the internal VCC regulator is used to initially bias the VCC pin. After the
output voltage is established, the VCC pin bias current is supplied through the VCCX pin, which effectively
disables the internal VCC regulator. Any voltage greater than 4.0 V can supply VCC bias through the VCCX pin.
However, the voltage applied to the VCCX pin should never exceed 15 V. The voltage supplied through VCCX
must be large enough to drive the switching MOSFETs into full saturation.
Figure 26. VCC Bias from VOUT 4 V < VOUT < 15 V
Figure 27. VCC Bias with Additional Bias Supply
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