1.5A, 280kHz, Boost Regulator
LM5171
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
The LM5171 can be activated by either connecting the VCC pin to a voltage source or by enabling the SS pin.
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.
When the VCC voltage is below the minimum supply voltage, the VSW pin is in high impedance. Therefore,
current conducts directly from the input power source to the output through the inductor and diode. Once VCC
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.
When the VC pin voltage rises above the threshold, the internal power switch starts to switch and a voltage
pulse can be seen at the VSW pin. Detecting a low output voltage at the FB pin, the built−in frequency shift
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.
Dec. 2010 - Rev. 1.2.1
- 10 -
HTC
HTC [ HTC KOREA TAEJIN TECHNOLOGY CO. ]
