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
www.ti.com SNVS566G –APRIL 2008–REVISED FEBRUARY 2013
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 4.0V can supply VCC bias through the VCCX pin.
However, the voltage applied to the VCCX pin should never exceed 15V. The voltage supplied through VCCX
must be large enough to drive the switching MOSFETs into full saturation.
Figure 24. VCC Bias from VOUT 4V < VOUT < 15V
Figure 25. VCC Bias with Additional Bias Supply
PCB Layout and Thermal Considerations
In a buck-boost regulator, there are two loops where currents are switched very fast. The first loop starts from the
input capacitors, and then to the buck switch, the inductor, the boost switch then back to the input capacitor. The
second loop starts from the inductor, and then to the output diode, the output capacitor, the recirculating diode,
and back to the inductor. Minimizing the PC board area of these two loops reduces the stray inductance and
minimizes noise and the possibility of erratic operation. A ground plane in the PC board is recommended as a
means to connect the input filter capacitors to the output filter capacitors and the PGND pins of the LM5118.
Connect all of the low current ground connections (C
SS
, R
T
, C
RAMP
) directly to the regulator AGND pin. Connect
the AGND and PGND pins together through topside copper area covering the entire underside of the device.
Place several vias in this underside copper area to the ground plane of the input capacitors.
Copyright © 2008–2013, Texas Instruments Incorporated Submit Documentation Feedback 29
Product Folder Links: LM5118 LM5118-Q1