lm5118mhx National Semiconductor Corporation, lm5118mhx Datasheet - Page 11

• Manufacturer Part Number: lm5118mhx
• Description: Wide Voltage Range Buck-boost Controller
• Manufacturer: National Semiconductor Corporation

Operation Modes
Figure 4 illustrates how duty cycle affects the operational
mode and is useful for reference in the following discussions.
Initially, only the buck switch is active and the buck duty cycle
increases to maintain output regulation as VIN decreases.
When VIN is approximately equal to 15.5V, the boost switch
begins to operate with a low duty cycle. If VIN continues to
fall, the boost switch duty cycle increases and the buck switch
duty cycle decreases until they become equal at VIN = 13.2V.
Buck Mode Operation: VIN > VOUT
The LM5118 buck-boost regulator operates as a conventional
buck regulator with emulated current mode control while VIN
is greater than VOUT and the buck mode duty cycle is less
than 75%. In buck mode, the LO gate drive output to the boost
switch remains low.
FIGURE 4. Mode Dependence on Duty Cycle (VOUT =12V)
FIGURE 3. Buck-Boost Mode Operation
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Buck-Boost Mode Operation: VIN
When VIN decreases relative to VOUT, the duty cycle of the
buck switch will increase to maintain regulation. Once the duty
cycle reaches 75%, the boost switch starts to operate with a
very small duty cycle. As VIN is further decreased, the boost
switch duty cycle increases until it is the same as the buck
switch. As VIN is further decreased below VOUT, the buck
and boost switch operate together with the same duty cycle
and the regulator is in full buck-boost mode. This feature al-
lows the regulator to transition smoothly from buck to buck-
boost mode. It should be noted that the regulator can be
designed to operate with VIN less than 4 volts, but VIN must
be at least 5 volts during start-up. Figure 5 presents a timing
illustration of the gradual transition from buck to buck-boost
mode when the input voltage ramps downward over a few
switching cycles.
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VOUT
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