LM5118MHX/NOPB National Semiconductor, LM5118MHX/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM5118MHX/NOPB
• Description: IC CTLR BUCK-BOOST 20-TSSOP
• Manufacturer: National Semiconductor
• Series: PowerWise®r
• Type: Step-Down (Buck), Step-Up (Boost)r

Specifications of LM5118MHX/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.23 ~ 70 V
• Frequency - Switching: 500kHz
• 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
• For Use With: LM5118EVAL - BOARD EVALUATION FOR LM5118
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Power - Output: -
• Other names: LM5118MHX

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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
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
shows the basic operation of buck-boost mode. In
FIGURE 2. Buck Mode Operation
10
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.
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.
Figure 2
shows the basic operation of the LM5118
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