LT3482 LINER [Linear Technology], LT3482 Datasheet - Page 6

• Manufacturer Part Number: LT3482
• Description: 90V Boost DC/DC Converter with APD Current Monitor
• Manufacturer: LINER [Linear Technology]

OPERATIO
LT3482
FU CTIO AL DIAGRA
The LT3842 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 Functional Diagram. At the start of each
oscillator cycle, the SR latch is set, which turns on the
power switch, Q1. 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,
A2. When this voltage exceeds the level at the negative
input of A2, the SR latch is reset turning off the power
switch. The level at the negative input of A2 is set by the
error amplifi er A1, and is simply an amplifi ed version of the
difference between the feedback voltage and the reference
voltage of 1.235V, or externally provided CTRL voltage.
In this manner, the error amplifi er sets the correct peak
6
V
IN
U
R
S
C
S
10 GND
12 SHDN
9 GND
REFERENCE
1.235V
U
U
13
CTRL
CONTROL
BLOCK
–
+
+
A1
EAMP
14
FB
W
650kHz/1.1MHz
OSCILLATOR
GENERATOR
R
C
C
C
RAMP
Σ
–
+
COMPARATOR
A2
15
PWM
f
SET
R
S
current level to keep the output in regulation. If the error
amplifi er’s output increases, more current is delivered to
the output; if it decreases, less current is delivered.
The LT3482 has an integrated high side APD current moni-
tor with a 5:1 ratio. The MONIN pin can accept a supply
voltage up to 90V, 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 R3. This voltage signal can be used to
drive an external control block to adjust the APD voltage
by adjusting the feedback threshold of EAMP A1 through
the CTRL input.
V
Q
AMPLIFIER
CURRENT
IN
SENSE
V
16
11
IN
DRIVER
R3
MON
L1
SW
+
–
7
C4
8
Q1
SW
C
D1
FLY
6
CURRENT
MIRROR
PUMP
APD
D2
D3
MONIN
V
V
OUT2
OUT1
APD
NC
3482 BD
4
5
3
2
1
C2
C1
R1
R2
FB
C3
C
PL
3482fa