LM2841 datasheet(7/19 Pages) TI1

7 / 19 page

Max Duty

Cycle Limit

OSC

LIMIT

SET

PWM

Comp

RESET

FET

Driver

BUCK

DRIVE

Error

Amp

Bandgap

Soft

Start

Thermal

Shutdown

TSD

SHDN

Inductor

Current

Measurement

GND

UVLO

Comp

UVLO

Voltage

Regulator

VIN

LM2841, LM2842

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SNVS540H – MARCH 2009 – REVISED APRIL 2013

BLOCK DIAGRAM

Figure 13. Block Diagram

OPERATION

PROTECTION

The LM2840/1/2 has dedicated protection circuitry running during normal operation to protect the IC. The thermal

shutdown circuitry turns off the power device when the die temperature reaches excessive levels. The UVLO

comparator protects the power device during supply power startup and shutdown to prevent operation at

voltages less than the minimum input voltage. A gate drive (CB) under-voltage lockout is included to ensure that

there is enough gate drive voltage to drive the MOSFET before the device tries to start switching. The

LM2840/1/2 also features a shutdown mode decreasing the supply current to approximately 16 µA.

CONTINUOUS CONDUCTION MODE

The LM2840/1/2 contains a current-mode, PWM buck regulator. A buck regulator steps the input voltage down to

a lower output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady

state), the buck regulator operates in two cycles. The power switch is connected between VIN and SW. In the first

cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and

the load current is supplied by COUT and the rising current through the inductor. During the second cycle the

transistor is open and the diode is forward biased due to the fact that the inductor current cannot instantaneously

change direction. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of

these two cycles determines the output voltage. The output voltage is defined approximately as: D=VOUT/VIN and

D’ = (1-D) where D is the duty cycle of the switch. D and D' will be required for design calculations.

DESIGN PROCEDURE

This section presents guidelines for selecting external components.

SETTING THE OUTPUT VOLTAGE

The output voltage is set using the feedback pin and a resistor divider connected to the output as shown on the

front page schematic. The feedback pin voltage 0.765V, so the ratio of the feedback resistors sets the output

voltage according to the following equation: VOUT=0.765V(1+(R1/R2))

Typically R2 will be given as 100

Ω-10 kΩ for a starting value. To solve for R1 given R2 and VOUT use

R1=R2((VOUT/0.765V)-1).

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