LT3599
9
3599ff
For more information www.linear.com/LT3599
operaTion
The LT3599 uses a constant-frequency, peak 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.
To turn on the LT3599, the V
IN
pin must exceed 3.1V and
the SHDN/UVLO pin must exceed 1.4V. The SHDN/UVLO
pin threshold allows programming of an undervoltage
lockout (UVLO) threshold for the system input supply us
-
ing a simple resistor divider. A 4µA current flows into the
SHDN/
UVLO pin before the part turns on and is removed
after the part turns on. This current hysteresis allows the
programming of hysteresis for the UVLO threshold. See
“Shutdown Pin and Programming Undervoltage Lockout”
in the Applications Information section. For part switching,
the PWM pin must exceed 1V (typical). For micropower
shutdown, the SHDN/UVLO pin at 0V reduces V
IN
supply
current to approximately ~0µA.
LT3599 has a built-in boost converter which converts the
input voltage to a higher output voltage for driving 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 LED4 voltages are monitored for
output voltage regulation. During normal operation, when
all LEDs are used, the lowest LED pin voltage (LED1 to
LED4) is used to regulate the output voltage to ensure all
LED strings have enough voltage to run the programmed
current.
If the user prefers only three strings, then LED string 4 can
be disabled through the DISABLE4 pin and by connecting
LED4 to any other LED pin. If the user prefers only two
strings, then two pins are connected in parallel (i.e., LED1,2
and LED3,4 can be connected together in operation).
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) is proportional to the sum of the switch
current and oscillator ramp. When this signal exceeds the
V
C
voltage, 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 flows via the
Schottky diode charging the output capacitor. 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 either PWM dim
-
ming or analog dimming. PWM dimming is achieved by
pulsing the LED current using the PWM pin
. For constant
color LED dimming, the LT3599 provides up to a 3000:1
wide PWM dimming range by allowing the duty cycle of the
PWM pin to be reduced from 100% to as low as 0.033%.
When the PWM pin is low, switching is disabled and the
error amplifier 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.
In applications where the user can sacrifice OPENLED,
SHORTLED fault flag diagnostics, the dimming ratios can
be as high as 3000:1. Analog dimming of LED currents is
accomplished by varying the level of CTRL pin voltage.
This method, however, changes LED color since dimming
is achieved by changing LED current. For CTRL pin voltage
less than 1V, LED current is defined as:
I
LED
= V
CTRL
•
1330
R
ISET
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Amps
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The LT3599 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 LED1-4 pin voltage is below 0.3V, the string is treated
as an open LED string. As a result, an OPENLED flag is
set. If a LED string is opened during regular operation,
the output voltage will regulate to the optimum voltage
for the remaining connected strings.
If a short occurs between V
OUT
and any LED pin during
operation, the LT3599 immediately turns off the LED pin
current of the shorted string and sets a SHORTLED flag.
Disabling the LED pin current protects the LT3599 from high
power thermal dissipation and ensures reliable operation.
