1.5A, 280kHz, Boost Regulator

LM5171

Dec. 2010 - Rev. 1.2.1

- 10

-

HTC

Short Circuit Condition

When a short circuit condition happens in a boost circuit, the inductor current will increase during the whole

switching cycle, causing excessive current to be drawn from the input power supply. Since control ICs don’t

have the means to limit load current, an external current limit circuit (such as a fuse or relay) has to be

implemented to protect the load, power supply and ICs.

In other topologies, the frequency shift built into the IC prevents damage to the chip and external components.

This feature reduces the minimum duty cycle and allows the transformer secondary to absorb excess energy

before the switch turns back on.

Figure22. Startup Waveforms of Circuit Shown in the Application Circuit. Load = 400mA

Startup waveforms shown in Figure 22 are measured in the boost converter demonstrated in the Application

circuit on the page 4 of this document. Recorded after the input voltage is turned on, this waveform shows the

various phases during the power up transition.

reaches approximately 1.5V, the internal power switch briefly turns on. This is a part of the LM5171 normal

operation. The turn−on of the power switch accounts for the initial current swing.

feature reduces the switching frequency to a fraction of its nominal value, reducing the minimum duty cycle,

which is otherwise limited by the minimum on−time of the switch. The peak current during this phase is

clamped by the internal current limit.

When the FB pin voltage rises above 0.4V, the frequency increases to its nominal value, and the peak current

begins to decrease as the output approaches the regulation voltage. The overshoot of the output voltage is

prevented by the active pull−on, by which the sink current of the error amplifier is increased once an

overvoltage condition is detected. The overvoltage condition is defined as when the FB pin voltage is 50mV

greater than the reference voltage.