ОПИСАНИЕ

The LT3571 is a current mode step-up DC/DC converter designed to bias avalanche photodiodes (APDs) in optical receivers with an output voltage up to 75V. The LT3571 features a high side fixed voltage drop APD current monitor with better than 10% relative accuracy over the entire temperature range. The integrated power switch, Schottky diode and APD current monitor allow a small solution footprint and low solution cost. It combines a traditional voltage loop and a unique current loop to operate as a constant-current source or constant-voltage source. The inductor-based topology ensures an input free from switching noise. The integrated high side current monitor produces a current proportional to the APD current with better than 10% relative accuracy over four decades of dynamic range in the input range of 250nA to 2.5mA. This current can be used as a reference to provide a digitally programmed output voltage via the CTRL pin. The LT3571 is available in the tiny footprint (3mm × 3mm) 16-lead QFN package.

 

ОСОБЕННОСТИ

High APD Voltage: Up to 70V

Integrated Schottky Diode

75V, 370mA Internal Switch

High Side Fixed Voltage Drop APD Current Monitor

Adjustable Frequency: 250kHz to 2MHz

Синхронизация по частоте

Wide VIN Range: 2.7V to 20V

Constant-Voltage and Constant-Current Regulation

Programmable Current Limit Protection

Surface Mount Components

Low Shutdown Current <1μA

Internal Soft-Start

Internal Compensation

CTRL Pin Allows Output Adjustment with No Polarity Inversion

3mm × 3mm 16-Lead QFN Package

 

ПРИЛОЖЕНИЯ

APD Bias

PIN Diode Bias

Optical Receivers and Modules

Fiber Optic Network Equipment

 

ОПЕРАЦИЯ

The LT3571 boost converter uses a constant-frequency current mode control scheme to provide excellent line and load regulation. At the start of each oscillator cycle, the SR latch is set, which turns on the Q1 power switch. A voltage proportional to the switch current is added to a stabilizing ramp and the resulting sum is fed into the positive terminal of the PWM comparator, A4. When this voltage exceeds the level at the negative input of A4, the SR latch is reset, turning off the power switch. The level at the negative input of A4 is set by the error amplifier A3. A3 has two inputs, one from the voltage feedback loop and the other one from the current loop. Whichever feedback input is lower takes precedence and forces the converter into either constant-current or constant-voltage mode. The LT3571 is designed to transition cleanly between these two modes of operation. The current sense amplifier senses the voltage across RSENSE and provides a pre-gain to amplifier A1. The output of A1 is simply an amplified version of the difference between the voltage across RSENSE and 200mV.

The LT3571 has an integrated high side APD current monitor with a 5:1 ratio. The voltage drop across the MONIN pin and APD pin is fixed at 5V. The MONIN pin can accept a supply voltage up to 75V, which is suitable for APD photodiode applications. The MON pin has an open-circuit protection feature and is internally clamped to 11.5V.

 

ИНФОРМАЦИЯ О ПРИЛОЖЕНИЯХ

Switching Frequency

There are two methods to set the switching frequency of the LT3571. Both methods require a resistor connected at the RT pin. Do not leave the RT pin open. Also, do not load this pin with a capacitor. A resistor must always be connected for proper operation. One way to set the frequency is simply connecting an external resistor between the RT pin and GND. The other way is to make the LT3571 synchronize with an external clock via the SYNC pin. For proper operation, a resistor should be connected at the RT pin and able to generate a switching frequency 20% lower than the external clock when the external clock is absent.

APD Current Monitor

The power supply switching noise associated with a switching power supply can interfere with the photodiode DC measurement. To suppress this noise, a 0.1μF capacitor is recommended at the APD pin. An additional series resistor is necessary to ensure enough high frequency compensation at the APD pin over the full operating range of the LT3571.An additional output lowpass filter, a 10k resistor and a 10nF capacitor in parallel at MON pin can further reduce the power supply noise, and other wide band noise, which might limit the measurement accuracy of low current levels.