Linear Technology DC1930A Demo Manual

DESCRIPTION
Demonstration circuit 1930A is a high current synchronous
inverting buck-boost LED driver featuring the
unique drive stage used on the LT3744 allows the anodes of
three LEDs to be connected together for better heat sinking
in both the step-down configuration and the inverting buck-
boost configuration. The pros and cons of each configuration
can be found in the data sheet. This demonstration circuit
1930A is for customers to test the inverting buck-boost
configuration. The step-down configuration is shown in a
separate demonstration circuit DC2339A.
The components are optimized for the efficiency, thermal
and PWM dimming for a 12V input. Each of the three outputs
is up to 5V, 12A with a 6.05V maximum output voltage
limit. The PWM1, PWM2 and PWM3 pins are set to low
by default. A DC or PWM signal is required to connect to
at least one of the PWM pins to enable the circuit. At any
giving time, output current only passes through one LED
determined by settings of PWM pins.
The CTRL1, CTRL2 and CTRL3 pins can be adjusted to
provide accurate analog dimming down to 20:1 ratio.
The input voltage range for the LT3744 to operate is from
3.3V to 36V. However, to maintain the output current
regulation in an inverting buck-boost regulator, the actual
input voltage range is determined by the load current,
voltage, the maximum duty cycle, etc. For a 5V, 12A load,
the demo board operating input range should limit to 7V
to 30V at room temperature. The load to be used with this
demo board is high current LEDs or laser diodes. Smaller
PERFORMANCE SUMMARY
SYMBOL PARAMETER
PV * Input Supply Voltage
IN
+ –
V to V Maximum Output Voltage
LED LED
I Output Current
OUT
F Switching Frequency
SW
EFF Efficiency at DC
*The board operating voltage range is narrower than the IC operating range in an inverting buck-boost configuration.
DEMO MANUAL DC1930A
High Current Synchronous
Inverting Buck-Boost LED Driver
LEDs may not be able to handle the high current, even for
LT 3744. The a short period of time. It is necessary to mount the LED
®
load on a proper heat sink. A fan may become necessary
to avoid exceeding LED's maximum temperate rating.
The typical efficiency of the demo board is 89.3% from a
12V input to 4V, 12A load. If an efficiency measurement is
needed in an application, the output voltage must be mea-
sured at the output capacitors instead of the LED load. This
prevents cable loss from being counted as loss of the board.
The demo circuit DC1930A achieves fast current rise time
from 0A to 12A in 5μs. To see the real rise time, wires
between the LED and the board should be as short as
possible to minimize the wire inductance and resistance. It
is recommended to measure the voltage across R32 with
a short 50Ω coax cable directly into a BNC connector on
the oscilloscope. Figure 1 shows the current rise time. The
current can be calculated from the measured voltage. A
current probe adds more delays to the rise time so using
a current probe is not recommended unless rise time is
not a concern.
The LT3744 data sheet gives a complete description of
the part, operation and application information. The data
sheet must be read in conjunction with this quick start
guide for demo circuit 1930A.
Design files for this circuit board are available at
http://www.linear.com/demo/DC1930A
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Specifications are at T
A
CONDITIONS
LED Voltage = 5V
CTRL1, CTRL2, CTRL3 = 1.5V
V = 12V, I = 12A, V
IN OUT
= 25°C
MIN
7
11.4
270
= 4V
OUT
LT3744
TYP MAX UNITS
30 V
6.05 V
12 12.6 A
300 330 kHz
89.3 %
dc1930af
1