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User's Guide
SNVA090B–May 2004–Revised April 2013
AN-1330 LM5008 Evaluation Board
1 Introduction
The LM5008 evaluation board is designed to provide the design engineer with a fully functional step down
switching regulator to evaluate LM5008 regulator IC, in a typical environment.
The performance of evaluation board is as follows:
• Input range: 12V to 95V, transients upto 100V (absolute maximum)
• Output voltage: 10V
• Output current: 300mA
• Measured efficiency: 89% at 200mA and V
IN
= 30V
• Board size: 31.9mm x 44.4mm
2 Theory of Operation
The LM5008 step-down switching regulator features all of the functions needed to implement low cost,
efficient, Buck bias regulators. This high voltage regulator contains an 100V, 0.5A N-channel Buck switch.
The regulator is based on hysteretic control scheme using an on-time inversely proportional to input
voltage (V
IN
). This feature allows the operating frequency to remain relatively constant with load and input
voltage variations. The hysteretic control requires no control loop compensation, while providing fast load
transient response. Additional protection features include : Thermal Shutdown, V
CC
undervoltage lockout
and maximum duty–cycle limiter. LM5008 can be used in numerous applications to efficiently regulate step
down higher voltage inputs. This regulator is well suited for 48V telecom and the new 42V automotive
power bus ranges.
The LM5008 operates in discontinuous conduction mode at light load currents or continuous conduction
mode at heavier load currents. In discontinuous conduction mode, current through the output inductor
starts at zero and ramps up to the peak value during the buck switch on time, and then back to zero
during buck switch off time. In discontinuous conduction mode, the operating frequency can be relatively
low and will vary with load. Therefore at light loads the conversion efficiency is maintained , since
switching losses decrease with reduction in switching frequency.
The evaluation board just needs one high voltage power-supply (upto 100V and current rating of 0.5A).
The maximum output power is 3 watts. Either an electronic load or resistor bank can be used for testing
evaluation board.
3 Start-up Feature
There is no soft-start feature in LM5008, and it normally comes up with full load current to supply the
power demanded by the load within a short time. The LM5008 contains an intelligent current limit OFF
timer intended to reduce the foldback characteristic inherent with fixed off-time over-current protection. If
the current in the Buck FET exceeds 550mA the present cycle is immediately terminated (cycle by cycle
current limit). Following the termination of the cycle a non-resetable current limit off timer is initiated. The
duration of off time is a function of the external setting resistor (Rcl) and the FB pin voltage. When FB pin
voltage = 0V the current limit off time is internally preset to 35µs. This condition occurs in short circuit
operation when a maximum amount of off time is required.
In cases of overload (not complete short circuit) the current limit off time can be reduced as a function of
the output voltage (measured at the FB pin). Reducing the off time during smaller overloads reduces the
amount of foldback and also reduces the initial start-up time.
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SNVA090B–May 2004–Revised April 2013 AN-1330 LM5008 Evaluation Board
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