LT1618EMS Linear Technology, LT1618EMS Datasheet - Page 7

• Manufacturer Part Number: LT1618EMS
• Description: IC DC/DC CONV STEP-UP 10-MSOP
• Manufacturer: Linear Technology
• Type: Step-Up (Boost)r

Specifications of LT1618EMS

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.26 ~ 36 V
• Current - Output: 1.5A
• Frequency - Switching: 1.4MHz
• Voltage - Input: 1.6 ~ 18 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIONS
Setting Output Voltage
To set the output voltage, select the values of R1 and R2
(see Figure 1) according to the following equation.
For current source applications, use the FB pin for over-
voltage protection. Pick R1 and R2 so that the output
voltage will not go too high if the load is disconnected or
if the load current drops below the preset value. Typically
choose R1 and R2 so that the overvoltage value will be
about 20% to 30% higher than the normal output voltage
(when in constant-current mode). This prevents the volt-
age loop from interfering with the current loop in current
source applications. For battery charger applications, pick
the values of R1 and R2 to give the desired end of charge
voltage.
Selecting R
Use the following formula to choose the correct current
sense resistor value (for constant current operation).
For designs needing an adjustable current level, the I
pin is provided. With the I
nominal current sense voltage is 50mV (appearing be-
tween the ISP and ISN pins). Applying a positive DC
voltage to the I
voltage according to the following formula:
For example, if 1V is applied to the I
sense voltage will be reduced to about 18mV. This
adjustability allows the regulated current to be reduced
without changing the current sense resistor (e.g. to adjust
brightness in an LED driver or to reduce the charge current
in a battery charger). If the I
R
R
V
ISENSE
SENSE
1
=
R
2
= 50mV/I
⎛
⎜
⎝
=
1 263
V
SENSE
1 263
.
OUT
.
ADJ
/Current Sense Adjustment
–
V
MAX
U
1
pin will decrease the current sense
– ( . )
⎞
⎟
⎠
25
0 8
INFORMATION
U
V
ADJ
IADJ
ADJ
pin is taken above 1.6V,
pin tied to ground, the
W
ADJ
pin, the current
U
ADJ
the output of the error amplifier (the V
down and the LT1618 will stop switching.
A pulse width modulated (PWM) signal can also be used
to adjust the current sense voltage; simply add an RC
filterto convert the PWM signal into a DC voltage for the
I
ground. Do not leave the pin floating.
For applications needing only a simple one-step current
sense adjustment, the circuit in Figure 2 works well. If a
large value resistor (≥2MΩ) is placed between the I
and ground, the current sense voltage will reduce to about
25mV, providing a 50% reduction in current. Do not leave
the I
current value in both states, and is controlled by a logic
signal without the need for a variable PWM or DC control
signal. When the NMOS transistor is on, the current sense
voltage will be 50mV, when it is off, the current sense
voltage will be reduced to 25mV.
Considerations When Sensing Input Current
In addition to regulating the DC output current for current-
source applications, the constant-current loop of the
LT1618 can also be used to provide an accurate input
current limit. Boost converters cannot provide output
short-circuit protection, but the surge turn-on current can
be drastically reduced using the LT1618’s current sense
at the input. SEPICs, however, have an output that is DC-
isolated from the input, so an input current limit not only
helps soft-start the output but also provides excellent
short-circuit protection.
ADJ
pin. If the I
ADJ
CURRENT
pin open. This method gives a well-regulated
FULL
ADJ
pin is not used, it should be tied to
Figure 2
2M
I
ADJ
LT1618
C
pin) will be pulled
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LT1618
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ADJ
7
pin