Operation

CONTINUOUS CONDUCTION MODE

The LM3224 is 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 regu-

lator 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

C

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

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 1.26V,

so the ratio of the feedback resistors sets the output voltage

according to the following equation:

SOFT-START CAPACITOR

The LM3224 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, Css. The soft-start time can be estimated

as:

Tss = Css*1.24V/Iss

THERMAL SHUTDOWN

The LM3224 includes thermal shutdown protection. If the die

temperature exceeds 140˚C the regulator will shut off the

power switch, significantly reducing power dissipation in the

device. The switch will remain off until the die temperature is

reduced to approximately 120˚C. If the cause of the excess

heating is not removed (excessive ambient temperature,

excessive power dissipation, or both) the device will con-

tinue to cycle on and off in this manner to protect from

damage.

20097602

FIGURE 1. Simplified Boost Converter Diagram

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

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