LT3598
9
3598fb
OPERATION
The LT3598 uses a constant-frequency, peak current mode
control scheme to provide excellent line and load regulation.
Each string can drive up to 30mA with 1.5% matching ac-
curacy between strings. Operation can be best understood
by referring to the Block Diagram in Figure 1.
LT3598 has a built-in boost converter which converts the
input voltage to a higher output voltage to drive LEDs.
The LED strings are connected to current sources where
the current level is set with an external resistor on the
I
SET
pin. The LED1 to LED6 voltages are monitored for
output voltage regulation. During normal operation, when
all LEDs are used, the lowest LED pin voltage (LED1 to
LED6) is used to regulate the output voltage to ensure all
LED strings have enough voltage to run the programmed
current.
For any unused LED strings, tie their LED pins to V
OUT
.
An unused LED string is no longer in the regulation loop,
nor does it affect open LED detection. Never allow unused
LED strings to be left open.
The basic loop uses a pulse from an internal oscillator
to set the SR latch and turn on the internal power NPN
switch Q1. The signal at the noninverting input of the PWM
comparator (A2 slope) is proportional to the sum of the
switch current and oscillator ramp. When slope exceeds
V
C
(the output of the g
m
amplifi er), the PWM comparator
resets the latch. The switch is then turned off, causing the
inductor current to lift the SW pin and turn on an external
Schottky diode connected to the output. Inductor current
fl ows via the Schottky diode charging the output capaci-
tor. The switch is turned on again at the next reset cycle
of the internal oscillator. During normal operation, the V
C
voltage controls the peak switch current limit and, hence,
the inductor current available to the output LEDs.
Dimming of the LEDs is accomplished by pulsing the LED
current using the PWM pin. When the PWM pin is low,
switching is disabled and the error amplifi er is turned off
so that it does not drive the V
C
pin. Also, all internal loads
on the V
C
pin are disabled so that the state of the V
C
pin
is maintained on the external compensation capacitor.
This feature reduces transient recovery time. When the
PWM input again transitions high, the peak switch current
returns to the correct value.
The LT3598 uses the FB pin to provide overvoltage protec-
tion when all LED strings are open. There is an internal
PMOS switch between V
OUT
and V
O_SW
that is controlled
by the PWM signal. During the PWM off-period, this
PMOS is turned off, allowing for higher dimming range
and lower current during shutdown. A resistor divider is
connected between the V
O_SW
pin and ground, which sets
the overvoltage protection voltage.
If the LED1-6 pin voltage is below 0.2V (for a certain delay
after 80% of the programmed output voltage is reached),
the string is treated as an open LED string. As a result,
OPENLED fl ag is set. If a LED string is open in the middle
of the operation, the regulation will continue.
OPENLED detection is disabled during the start-up phase
to avoid erratic fl ag generation. An LED string that is
disabled by connecting its LED pin to V
OUT
is not an open
LED condition. During normal operation, if an LED string
is open and has the lowest LED pin voltage, the output
voltage will regulate itself to fi nd another LED string that has
the lowest LED pin voltage at about 0.8V. If the open LED
string has an LED voltage above 0.8V, the output voltage
will remain the same. When the LED string is open, it is
no longer in the regulation loop. The OPENLED detection
is active only when the PWM signal is enabled. To avoid
spurious OPENLED detection and high PWM dimming
ratio, more output capacitance is recommended to allow
less voltage drop on V
OUT
.
During start-up, 10μA of current charges the external
soft-start capacitor. The SS pin directly limits the rate of
voltage rise on the V
C
pin, which in turn, limits the peak
switch current. Soft-start also enables the switching
frequency foldback to provide a clean start-up for the
LT3598. Current limit protects the power switch and
external components.
