MAX1848EKA-T Maxim Integrated Products, MAX1848EKA-T Datasheet - Page 6

• Manufacturer Part Number: MAX1848EKA-T
• Description: LED Drivers HIgh Efficiency 13V Step-Up Converter fo
• Manufacturer: Maxim Integrated Products

Specifications of MAX1848EKA-T

• Number Of Segments: 3
• Operating Supply Voltage: 2.6 V to 5.5 V
• Maximum Power Dissipation: 777 mW
• Maximum Operating Temperature: + 85 C
• Mounting Style: SMD/SMT
• Package / Case: SOT-23
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The MAX1848’s high efficiency and small size make it
ideally suited to drive series-connected LEDs. It oper-
ates as a boost DC-DC converter that controls output
current rather than voltage. The MAX1848 provides
even illumination by sending the same output current
through each LED, eliminating the need for expensive
factory calibration. The fast 1.2MHz internal oscillator
allows for a small inductor and small input and output
capacitors while minimizing input and output ripple.
The single analog control input allows easy adjustment
of LED brightness and on/off control. This allows either
simple logic-level on/off control or a DAC to control
both brightness and on/off. In shutdown, supply current
is reduced to a low 0.3µA. A programmable soft-start
gradually illuminates the LEDs, reducing the inrush cur-
rent during startup.
The MAX1848 attains soft-start by charging C
gradually with a constant 12µA current. When V
rises above 1.25V, the internal MOSFET begins switch-
ing, but at a reduced duty cycle. When V
above 2.25V, the duty cycle will be at its maximum.
The maximum startup time is determined by the value
of C
White LED Step-Up Converter in SOT23
Figure 1. Functional Diagram
6
CTRL
_______________________________________________________________________________________
COMP
170mV
1.2MHz
. For every 0.01µF connected to COMP, the
CONTROL
PWM
Detailed Description
SLOPE
CURRENT
SENSE
617kΩ
MAX1848
SHUTDOWN
GND
V+
13.25V
50kΩ
Soft-Start
COMP
COMP
LX
PGND
OUT
CS
COMP
COMP
rises
startup time will increase by 0.833ms. The start time
can be calculated by:
The MAX1848 is put into shutdown when V
than 100mV. In shutdown, supply current is reduced to
0.3µA by powering down the entire IC except for the
CTRL voltage detection circuitry. C
discharged during shutdown, allowing the device to re-
initiate a soft-start whenever the device is enabled.
When in shutdown, the internal N-channel FET does not
switch, which leaves a current path between the input
and the LEDs through the boost inductor and Schottky
diode. The minimum forward voltage of the LED array
must exceed the maximum V+ to ensure that the LEDs
remain off in shutdown. Typical shutdown timing
characteristics are shown in the Typical Operating
Characteristics.
Overvoltage protection occurs when V
13.25V. The protection circuitry stops the internal MOS-
FET from switching and causes V
GND. The device comes out of overvoltage lockout and
into soft-start when V
Adjusting the MAX1848’s output current will change the
brightness of the LEDs. An analog input (CTRL) and the
sense resistor value set the output current. Output cur-
rent is given by:
The V
250mV to (V+ + 2V) or 5.5V, whichever is less. To set
the maximum current, calculate R
at its maximum. Power dissipation in R
less than 5mW; therefore, a standard chip resistor
is sufficient.
The exact values of input and output capacitors are not
critical. The typical value for the input capacitor is
3.3µF, and the typical value for the output capacitor is
1.0µF. Larger value capacitors can be used to reduce
input and output ripple, but at the expense of size and
higher cost.
CTRL
t
SOFT START (MAX)
voltage range for adjusting output current is
I
LED
-
=
OUT
13.33
falls below 12.25V.
Overvoltage Protection
V
Adjusting LED Current
CTRL
×
Design Procedure
=
Capacitor Selection
C
R
SENSE
COMP
SENSE
COMP
COMP
×
SENSE
12μ
when V
OUT
1
V
Shutdown
to decay to
CTRL
is passively
A
is typically
is above
CTRL
is less
is