LM3017_15 Datasheet, PDF(8/33 Page) Texas Instruments – High Efficiency Low-Side Controller with True Shutdown

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LM3017_15 Datasheet, PDF (8/33 Pages) Texas Instruments – High Efficiency Low-Side Controller with True Shutdown

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LM3017

SNOSC66C â MARCH 2012 â REVISED MARCH 2013

FUNCTIONAL BLOCK DIAGRAM

ISEN

Ramp Adjust

Pass

Control

Charge

Pump

1.27V

Reference

Limit

References

Soft

Start

Current

A Sense

Amp.

200 mV

Limit

Short-circuit

Comparator

Level

Shifter

PWM

Drive

Logic

VIN

Internal

Reg

www.ti.com

VCC

-E.A.

EN/MODE

Logic

OVP

Internal Slope

Compensation

OSC

EN/MODE

COMP

AGND

PGND

FUNCTIONAL DESCRIPTION

The LM3017 uses a fixed frequency, Pulse Width Modulated (PWM), current mode control architecture. A high-

side current sense amplifier provides inductor current information by sensing the voltage drop across RSEN. The

voltage across this resistor is fed into the ISEN pin. This voltage is then level shifted and fed into the positive input

of the PWM comparator. As with all architectures of this type, a compensation ramp is required to ensure stability

of the current control loop under all operating conditions. A nominal value of the ramp is provided internally while

additional ramp can be added through the ISEN pin. The output voltage is sensed through an external feedback

resistor divider network and fed into the error amplifier (EA) negative input (feedback pin, FB). The output of the

error amplifier (COMP pin) is added to the slope compensation ramp and fed into the negative input of the PWM

comparator.

At the start of any switching cycle, the oscillator sets a high signal on the DR pin (gate of the external MOSFET)

and the external MOSFET turns on. When the voltage on the positive input of the PWM comparator exceeds the

negative input, the Drive Logic is reset and the external MOSFET turns off.

Under extremely light load or no-load conditions, the energy delivered to the output capacitor when the external

MOSFET is on during the minimum on time is more than what is delivered to the load. An over-voltage

comparator inside the LM3017 prevents the output voltage from rising under these conditions by sensing the

feedback (FB pin) voltage and resetting the RS latch. The latch remains in a reset state until the output decays to

the nominal value. Thus the operating frequency decreases at light loads, resulting in excellent efficiency.

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