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Application Information

The

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 (BS) under-voltage lockout is included to guarantee that there is

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

also features

a shutdown mode decreasing the supply current to approximately 0.7

μA.

Continuous Conduction Mode

The

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 Co 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=Vo

/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 is 0.8V, so the ratio of the feedback resistors sets the output voltage

according to the following equation:

R1+R2

Vo = 0.8 *

R2

Typically R2 will be given as 100

Ω-10 KΩ for a starting value. To solve for R1 given R2 and Vo use :

Vo

R1=R2 *(

-1)

0.8V

Feedforward Capacitor Selection

Internal compensation function allows users saving time in design and saving cost by reducing the number of

external components. The use of a feedforward capacitor C1 in the feedback network is recommended to improve

the transient response or higher phase margin.