LM3310 datasheet(14/25 Pages) NSC

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Operation

CONTINUOUS CONDUCTION MODE

The LM3310 contains a current-mode, PWM boost regulator.

A boost regulator steps the input voltage up to a higher

output voltage. In continuous conduction mode (when the

inductor current never reaches zero at steady state), the

boost regulator operates in two cycles.

In the first cycle of operation, shown in Figure 1 (a), the

transistor is closed and the diode is reverse biased. Energy

is collected in the inductor and the load current is supplied by

OUT.

The second cycle is shown in Figure 1 (b). During this cycle,

the transistor is open and the diode is forward biased. 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:

where D is the duty cycle of the switch, D and D' will be

required for design calculations.

SETTING THE OUTPUT VOLTAGE (BOOST

CONVERTER)

The output voltage is set using the feedback pin and a

resistor divider connected to the output as shown in the

typical operating circuit. The feedback pin voltage is 1.263V,

so the ratio of the feedback resistors sets the output voltage

according to the following equation:

SOFT-START CAPACITOR

The LM3310 has a soft-start pin that can be used to limit the

inductor inrush current on start-up. The external SS pin is

used to tailor the soft-start for a specific application but is not

required for all applications and can be left open when not

needed. When used a current source charges the external

soft-start capacitor C

SS until it reaches its typical clamp

voltage, V

SS. The soft-start time can be estimated as:

SS =CSS*VSS/ISS

THERMAL SHUTDOWN

The LM3310 includes thermal shutdown. If the die tempera-

ture reaches 145˚C the device will shut down until it cools to

a safe temperature at which point the device will resume

operation. If the adverse condition that is heating the device

is not removed (ambient temperature too high, short circuit

conditions, etc...) the device will continue to cycle on and off

to keep the die temperature below 145˚C. The thermal shut-

down has approximately 20˚C of hysteresis. When in ther-

mal shutdown the boost regulator, Op-Amp, and GPM blocks

will all be disabled.

INPUT UNDER-VOLTAGE PROTECTION

The LM3310 includes input under-voltage protection (UVP).

The purpose of the UVP is to protect the device both during

start-up and during normal operation from trying to operate

with insufficient input voltage. During start-up using a ramp-

ing input voltage the UVP circuitry ensures that the device

20133302

FIGURE 1. Simplified Boost Converter Diagram

(a) First Cycle of Operation (b) Second Cycle Of Operation

LM3310 Datasheet(PDF) 14 Page - National Semiconductor (TI)

LM3310 Datasheet(HTML) 14 Page - National Semiconductor (TI)

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