Datasheet

VOUT

CSG

LM5118

VIN

Q2 (OFF)

Buck Switch

Current

Diode Current

LM5118, LM5118-Q1

SNVS566H –APRIL 2008–REVISED JANUARY 2014

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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 A. The regulator control method is

based on current mode control utilizing an emulated 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 reduces noise sensitivity of the pulse-width modulation circuit, allowing reliable processing of very small

duty cycles necessary in high input voltage applications. The operating frequency is user programmable from 50

kHz to 500 kHz. An oscillator synchronization pin allows multiple LM5118 regulators to self synchronize or be

synchronized to an external clock. Fault protection features include current limiting, thermal 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 state at the enable pin will put the regulator into an extremely low

current shutdown state. The device is available in the HTSSOP-20EP package featuring an exposed pad to aid

in thermal dissipation.

A buck-boost regulator can maintain regulation for input voltages either higher or lower than the output voltage.

The challenge is that buck-boost power converters are not as efficient 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 approach maintains regulation over a wide range of input voltages, while

maintaining the optimal conversion efficiency in the normal buck mode. The gradual transition between modes

eliminates disturbances at the output during transitions. Figure 9 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 proportion to the VIN - VOUT voltage difference when Q1 is active and ramps down through the recirculating

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.

Figure 9. Buck Mode Operation

Figure 10 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 (proportional to VIN) when Q1 and Q2 are active

and ramps down through the recirculating 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.

Product Folder Links: LM5118 LM5118-Q1