LT3513 LINER [Linear Technology], LT3513 Datasheet - Page 12

• Manufacturer Part Number: LT3513
• Description: 2MHz High Current 5-Output Regulator for TFT-LCD Panels
• Manufacturer: LINER [Linear Technology]

LT3513
All four switchers employ a constant-frequency current
mode control scheme. Switcher 1, the step-down regula-
tor, differs slightly from the others with inductor current
sense. Instead of monitoring the current at the switch,
current nodes are used to measure the current through
the inductor. Inductor current sense does not suffer from
minimum on-time problems, therefore always keep-
ing the switch current limited with any input-to-output
voltage ratio. Switcher 1 is always synchronized to the
master oscillator. The other three switchers each have
their own slave oscillator. The slave oscillator reduces the
frequency when the feedback voltage dips below 0.75V
and decreases linearly below the threshold as shown in
the Performance Characteristics’ Frequency Foldback plot.
Other than these two differences, the control loop is similar
in all four switchers. A pulse from the master oscillator
for switcher 1 or a pulse from the slave oscillator for the
other three switchers sets the RS latch and turns on the
internal NPN bipolar power switch. Current in the switch
and the external inductor begins to increase. When this
current exceeds a level determined by the voltage at V
current comparator resets the latch, turning off the switch.
The current in the inductor fl ows through the Schottky
diode and begins to decrease. The cycle begins again at
the next pulse from the oscillator. In this way, the voltage
on the V
the output. The internal error amplifi er regulates the output
by continually adjusting the V
for switching on the V
of 1.8V limits the V
contain an independent current limit not dependent on V
or duty cycle. Switcher 1’s current limit is controlled by
the V
ers also use slope compensation to ensure stability with
the current mode scheme at duty cycles above 50%. The
RUN-SS1, RUN-SS2 and RUN-SS3/4 pins control the rate
of rise of the feedback pins.
The switch driver for SW1 operates either from V
the BOOST pin. An external capacitor and an integrated
Schottky diode are used to generate a voltage at the BOOST
pin that is higher than the input supply. This allows the
driver to saturate the internal bipolar NPN power switch
for effi cient operation.
OPERATION
12
C
voltage and varies with duty cycle. All four switch-
C
pin controls the current through the inductor to
C
voltage. Switchers 2, 3 and 4 also
C
pin is 0.8V, and an active clamp
C
pin voltage. The threshold
IN
or from
C
, the
C
INPUT VOLTAGE RANGE STEP-DOWN CONSIDERATION
The minimum operating voltage of switcher 1 is determined
either by the LT3513’s undervoltage lockout of ~4V or by
its maximum duty cycle. A user defi ned undervoltage
lockout may be set with the UVLO pin at a voltage higher
than the internal undervoltage lockout. The duty cycle is
the fraction of time that the internal switch is on and is
determined by the input and output voltages:
where V
(~0.4V) and V
(~0.3V at maximum load). This leads to a minimum input
voltage of:
with DC
The user defi ned undervoltage is set by a resistor divider
connected to the UVLO pin. The comparator pulls 3μA
from the pin when the UVLO pin is higher than 1.25V.
The hysteresis and minimum input voltage equations are
as follows:
DC
V
V
V
IN MIN
HYS
IN MIN
(
(
=
MAX
F
V
=
IN
is the forward voltage drop of the catch diode
)
)
(
V
R
=
=
OUT
–
= 0.75.
2 2
1 25
DC
SW
V
+
.
SW
MAX
+
is the voltage drop of the internal switch
k
V
V
V
+
)
OUT
F
R
• .
V
–
V
3 9
1
F
IN
R
+
V
R2
R1
3513 A1
+
1
F
R
µA
V
+
2
F
V
SW
UVLO
38
3513fa