lp55271tlx National Semiconductor Corporation, lp55271tlx Datasheet - Page 7

• Manufacturer Part Number: lp55271tlx
• Description: Tiny Led Driver For Camera Flash And 4 Leds With I2c Programmability, Connectivity Test And Audio Synchronization
• Manufacturer: National Semiconductor Corporation

Magnetic Boost DC/DC Converter
The LP55271 boost DC/DC converter generates a 4.00 –
5.40V output voltage to drive the LEDs from a single Li-Ion
battery (3.0V to 4.5V). The output voltage is controlled with a
4-bit register in 8 steps. The converter is a magnetic switch-
ing PWM mode DC/DC converter with a current limit. The
converter has 2.0 MHz / 1.0 MHz selectable switching fre-
quency operation, when the timing resistor RT is 82 kΩ.
The LP55271 boost converter uses pulse-skipping elimina-
tion method to stabilize the noise spectrum. Even with light
load or no load a minimum length current pulse is fed to the
inductor. An internal active load is used to remove the ex-
cess charge from the output capacitor when needed.
The topology of the magnetic boost converter is called CPM
control, current programmed mode, where the inductor cur-
rent is measured and controlled with the feedback. The
output voltage control changes the resistor divider in the
feedback loop.
Boost Converter Topology
7
The following figure shows the boost topology with the pro-
tection circuitry. Four different protection schemes are imple-
mented:
1. Over voltage protection, limits the maximum output volt-
2. Over current protection, limits the maximum inductor
3. Feedback (FB) protection for no connection.
4. Duty cycle limiting, done with digital control.
age.
— Keeps the output below breakdown voltage.
— Prevents boost operation if battery voltage is much
current.
— Voltage over switching NMOS is monitored; too high
higher than desired output.
voltages turn the switch off.
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