lm5118mhx National Semiconductor Corporation, lm5118mhx Datasheet - Page 10

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

www.national.com
Detailed Operating Description
The LM5118 high voltage switching regulator features all of
the functions necessary to implement an efficient high voltage
buck or buck-boost regulator using a minimum of external
components. The regulator switches smoothly from buck to
buck-boost operation as the input voltage approaches the
output voltage, allowing operation with the input greater than
or less than the output voltage. This easy to use regulator
integrates high-side and low-side MOSFET drivers capable
of supplying peak currents of 2 Amps. The regulator control
method is based on current mode control utilizing an emulat-
ed current ramp. Peak current mode control provides inherent
line feed-forward, cycle-by-cycle current limiting and ease of
loop compensation. The use of an emulated control ramp re-
duces noise sensitivity of the pulse-width modulation circuit,
allowing reliable processing of very small duty cycles neces-
sary in high input voltage applications. The operating fre-
quency is user programmable from 50 kHz to 500 kHz. An
oscillator synchronization pin allows multiple LM5118 regula-
tors to self synchronize or be synchronized to an external
clock. Fault protection features include current limiting, ther-
mal shutdown and remote shutdown capability. An under-
voltage lockout input allows regulator shutdown when the
input voltage is below a user selected threshold, and a low
Figure 3 shows the basic operation of buck-boost mode. In
buck-boost mode both Q1 and Q2 are active for the same time
interval each cycle. The inductor current ramps up (propor-
tional to VIN) when Q1 and Q2 are active and ramps down
FIGURE 2. Buck Mode Operation
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state at the enable pin will put the regulator into an extremely
low current shutdown state. The device is available in the
TSSOP-20EP package featuring an exposed pad to aid in
thermal dissipation.
A buck-boost regulator can maintain regulation for input volt-
ages either higher or lower than the output voltage. The
challenge is that buck-boost power converters are not as ef-
ficient as buck regulators. The LM5118 has been designed as
a dual mode controller whereby the power converter acts as
a buck regulator while the input voltage is above the output.
As the input voltage approaches the output voltage, a gradual
transition to the buck-boost mode occurs. The dual mode ap-
proach maintains regulation over a wide range of input volt-
ages, while maintaining the optimal conversion efficiency in
the normal buck mode. The gradual transition between
modes eliminates disturbances at the output during transi-
tions. Figure 2 shows the basic operation of the LM5118
regulator in the buck mode. In buck mode, transistor Q1 is
active and Q2 is disabled. The inductor current ramps in pro-
portion to the Vin - Vout voltage difference when Q1 is active
and ramps down through the re-circulating diode D1 when Q1
is off. The first order buck mode transfer function is VOUT/
VIN = D, where D is the duty cycle of the buck switch, Q1.
through the re-circulating diode during the off time. The first
order buck-boost transfer function is VOUT/VIN = D/(1-D),
where D is the duty cycle of Q1 and Q2.
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