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LT3571

3571fa

The LT3571 boost converter uses a constant-frequency

current mode control scheme to provide excellent line

and load regulation. Operation can be best understood

by referring to the Block Diagram in Figure 1. 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.

In this manner, the error ampliﬁer sets the correct peak

switch current level to regulate through RSENSE. The FB

voltage loop is implemented by the ampliﬁer A2. When

the voltage loop dominates, the error ampliﬁer regulates

the FB pin to the lower of 1V, or externally provided CTRL

voltage (constant-voltage mode), and sets the correct peak

current level to keep the output in regulation.

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.

If an APD is tied to the APD pin, the current will be mir-

rored to the MON pin and converted to a voltage signal

by the resistor R4. This voltage signal can be used to

drive an external control block to adjust the APD voltage

by adjusting the feedback threshold of EAMP A2 through

the CTRL input.

OPERATION

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