LT3492
9
3492fa
APPLICATIONS INFORMATION
Operation
The LT3492 uses a fi xed frequency, current mode control
scheme to provide excellent line and load regulation. Op-
eration can be best understood by referring to the Block
Diagram in Figure 1. The oscillator, ramp generator, refer-
ence, internal regulator and UVLO are shared among the
three converters. The control circuitry, power switch etc.,
are replicated for each of the three converters. Figure 1
shows the shared circuits and only converter 1 circuits.
If the SHDN pin is logic low, the LT3492 is shut down
and draws minimal current from V
IN
. If the SHDN pin is
logic high, the internal bias circuits turn on. The switching
regulators start to operate when their respective PWM
signal goes high.
The main control loop can be understood by following
the operation of converter 1. The start of each oscillator
cycle sets the SR latch, A3, and turns on power switch
Q1. The signal at the noninverting input (SLOPE node)
of the PWM comparator A2 is proportional to the sum
of the switch current and oscillator ramp. When SLOPE
exceeds V
C
(the output of the error amplifi er A1), A2 resets
the latch and turns off the power switch Q1 through A4
and A5. In this manner, A10 and A2 set the correct peak
current level to keep the output in regulation. Amplifi er
A8 has two noninverting inputs, one from the 1V internal
voltage reference and the other one from the CTRL1 pin.
Whichever input is lower takes precedence. A8, Q3 and R2
force V1, the voltage across R1, to be one tenth of either
1V or the voltage of CTRL1 pin, whichever is lower. V
SENSE
is the voltage across the sensing resistor, R
SENSE
, which is
connected in series with the LEDs. V
SENSE
is compared to
V1 by A1. If V
SENSE
is higher than V1, the output of A1 will
decrease, thus reducing the amount of current delivered to
LEDs. In this manner the current sensing voltage V
SENSE
is regulated to V1.
Converters 2 and 3 are identical to converter 1.
PWM Dimming Control
The LED array can be dimmed with pulse width modulation
using the PWM1 pin and an external P-channel MOSFET,
M1. If the PWM1 pin is pulled high, M1 is turned on by
internal driver A7 and converter 1 operates nominally.
A7 limits ISP1-TG1 to 6.5V to protect the gate of M1. If
the PWM1 pin is pulled low, Q1 is turned off. Converter 1
stops operating, M1 is turned off, disconnects the LED
array and stops current draw from output capacitor C2. The
V
C1
pin is also disconnected from the internal circuitry and
draws minimal current from the compensation capacitor
C
C
. The V
C1
pin and the output capacitor store the state
of the LED current until PWM1 is pulled up again. This
leads to a highly linear relationship between pulse width
and output light, and allows for a large and accurate dim-
ming range. A P-channel MOSFET with smaller total gate
charge (Q
G
) improves the dimming performance, since
it can be turned on and off faster. Use a MOSFET with a
Q
G
lower than 10nC, and a minimum V
TH
of –1V to –2V.
Don’t use a Low V
TH
PMOS. To optimize the PWM control
of all the three channels, the rising edge of all the three
PWM signals should be synchronized.
In the applications where high dimming ratio is not required,
M1 can be omitted to reduce cost. In these conditions,
TG1 should be left open. The PWM dimming range can be
further increased by using CTRL1 pin to linearly adjust the
current sense threshold during the PWM1 high state.
Loop Compensation
Loop compensation determines the stability and transient
performance. The LT3492 uses current mode control to
regulate the output, which simplifi es loop compensation.
To compensate the feedback loop of the LT3492, a series
resistor-capacitor network should be connected from the
V
C
pin to GND. For most applications, the compensation
capacitor should be in the range of 100pF to 2.2nF. The com-
pensation resistor is usually in the range of 5k to 50k.
To obtain the best performance, tradeoffs should be made
in the compensation network design. A higher value of
compensation capacitor improves the stability and dim-
ming range (a larger capacitance helps hold the V
C
voltage
when the PWM signal is low). However, a large compen-
sation capacitor also increases the start-up time and the
time to recover from a fault condition. Similarly, a larger
compensation resistor improves the transient response
but may reduce the phase margin. A practical approach
is to start with one of the circuits in this data sheet that
