Datasheet

LT3476
6
3476fb
operAtion
The LT3476 is a constant-frequency, current mode regula-
tor with an internal power switch. Operation can be best
understood by referring to the Block Diagram. 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, 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 amplifier A1, and is simply an amplified
version of the difference between the voltage across the
internal resistor R
SET
and the voltage across the external
current sense resistor R
SNS
. In this manner, the error
amplifier sets the correct peak switch current level to
regulate the current through R
SNS
. If the error amplifier’s
output increases, more current is delivered to the output;
if it decreases, less current is delivered.
The current regulated in R
SNS
can be adjusted by chang-
ing the voltage across R
SET
using the V
ADJ
input pin. The
amplifier A4 regulates current in Q3 to produce a voltage
across R
SET
equal to V
ADJ
. This current flowing through
transistor Q3 also produces a voltage across R
SET
one-
tenth the magnitude of the V
ADJ
input and level shifted to
the CAP input. The voltage across R
SET
is limited to 125mV
(typ) by the separate 1.25V input on A4.
The average current regulated in R
SNS
can also be adjusted
for dimming using the PWM pin. When the PWM pin is
low, switching is disabled and the error amplifier is turned
off so that it does not drive the VC pin. Also, all internal
loads on the VC pin are disabled so that the charge state
of the VC pin will be saved on the external compensation
capacitor. This feature reduces transient recovery time
because when the PWM input again transitions high, the
demand current for the switch returns to the value just
before PWM last transitioned low.
–
+
–
+
–
+
+
–
+
V
IN
3V
3476 BD
PWM
16, 17,
34, 35
VC
1, 12, 13, 38
V
IN
REF
SHUTDOWN
RT
OVERVOLTAGE
DETECT
ERROR
AMPLIFIER
EXTERNAL COMPONENTS
BUCK MODE
CURRENT SENSE
AMPLIFIER
R
SW
0.02Ω
1.25V
1.05V
200kHz
to 2MHz
OSCILLATOR
RAMP
GENERATOR
R Q
S
PV
IN
33V
LT3476 CHANNEL
33
7
–
+
∑
R
SNS
(EXT)
0.1Ω
18
6
CAP
3, 4, 9, 10
V
ADJ
14, 15,
36, 37
R
SET1
20kΩ
R
SET
2kΩ
PWM
THERMAL
LIMIT
145°C
ISRC
300µA
Q3
A4
A2
A3
35V
LED
2, 5, 8, 11
C
BYP
2.2µF
C
FILT
0.1µF
10µH
25k
Q4
V1
IDLE MODE
A1
PWM
COMPARATOR
Q1
MAIN
SWITCH
SHDN
SW
22-29
NC
19, 20, 21
30, 31, 32
DRIVER
Q2
LED ARRAY
block DiAgrAM