lt3571 Linear Technology Corporation, lt3571 Datasheet - Page 8

• Manufacturer Part Number: lt3571
• Description: 75v Dc/dc Converter For Apd Bias
• Manufacturer: Linear Technology Corporation

LT3571
OPERATION
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 amplifi er 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 amplifi er senses the voltage
across R
The output of A1 is simply an amplifi ed version of the
difference between the voltage across R
8
SENSE
and provides a pre-gain to amplifi er A1.
SENSE
and 200mV.
In this manner, the error amplifi er sets the correct peak
switch current level to regulate through R
voltage loop is implemented by the amplifi er A2. When
the voltage loop dominates, the error amplifi er 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 fi xed 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.
SENSE
. The FB
3571f