LM5118Q1MHX/NOPB National Semiconductor, LM5118Q1MHX/NOPB Datasheet - Page 21

• Manufacturer Part Number: LM5118Q1MHX/NOPB
• Description: IC BUCK BST ADJ 20TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LM5118Q1MHX/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 70 V
• Frequency - Switching: 200kHz, 515kHz
• Voltage - Input: 3 ~ 75 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 20-TSSOP Exposed Pad, 20-eTSSOP, 20-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -
• Other names: LM5118Q1MHX

The plots shown in Figures 13, 14 and 15 illustrate the gain
and phase diagrams of the design example. The overall band-
width is lower in a buck-boost application due the compen-
sation challenges associated with the right-half-plane zero.
For a pure buck application, the bandwidth could be much
higher. The LM5116 datasheet is a good reference for com-
pensation design of a pure buck mode regulator.
Bias Power Dissipation Reduction
Buck or Buck-boost regulators operating with high input volt-
age can dissipate an appreciable amount of power while
supplying the required bias current of the IC. The VCC regu-
lator must step-down the input voltage VIN to a nominal VCC
level of 7V. The large voltage drop across the VCC regulator
FIGURE 16. VCC Bias from VOUT 4V < VOUT < 15V
FIGURE 15. Overall Loop Gain and Phase
21
translates into high power dissipation in the VCC regulator.
There are several techniques that can significantly reduce this
bias regulator power dissipation. Figures 16 and 17 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 4.0V can supply
VCC bias through the VCCX pin. However, the voltage ap-
plied 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.
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