LX1995-1CSE MICROSEMI, LX1995-1CSE Datasheet - Page 6

• Manufacturer Part Number: LX1995-1CSE
• Manufacturer: MICROSEMI

Specifications of LX1995-1CSE

• Operating Supply Voltage (typ): 1.8/2.5/3.3/5V
• Number Of Segments: 4
• Operating Temperature (min): -40C
• Operating Temperature (max): 85C
• Operating Temperature Classification: Industrial
• Package Type: SOT-23
• Pin Count: 5
• Mounting: Surface Mount
• Operating Supply Voltage (min): 1.6V
• Operating Supply Voltage (max): 5.5V
• Lead Free Status / Rohs Status: Compliant

Copyright © 2000
Rev. 1.1, 2005-07-07
O
driving a series of white or color LEDs. It operates in a
pseudo-hysteretic mode with a fixed, 300ns, switch “off
time”. When the LX1995-x enables, the LED current
decreases causing the FB voltage to decrease to a value less
than 320mV. The feedback comparator (See Simplified
Block Diagram) activates the control logic. The control
logic turns on the DRV output circuit that connects to the
internal N-Channel MOSFET gate.
(SW) is switched “on” and remains “on” until the inductor
current ramps up to the peak current level (typically 325mA
for LX1995-1).
output capacitor during the inductor charging cycle. Once
the peak inductor current value is achieved, the output is
turned off and the energy stored in the inductor delivers to
the load. This causes the voltage to rise across the current
setting resistor (R
LX1995-x continues to switch until the voltage at the FB
pin exceeds 320mV. The value of R
dividing 320mV by the maximum series LED current. A
minimum value of 3.3Ω is recommended for R
voltage at the FB pin is the product of the LED current
(I
D
Voltage.
SHDN pin (See Figure 1). This turns the LX1995-X on and
off which pulses the LED current between zero and the
setting determined by R
DC voltage supply to the FB pin. As the DC voltage
increases, the LED current decreases. The equation (see
Figure 3) is:
R =
I
LED
LED
IMMING
PERATING
1
The LX1995-x is a PFM boost converter optimized for
The LED load is powered from energy stored in the
LX1995-x supports two dimming methods: PWM or DC
PWM mode: Connect system PWM logic signal to the
DC Voltage mode: The designer can apply an adjustable
) and R
I
=
LED
TM
320
R
1
1
(
mV
MAX
M
⎡
⎢
⎢
⎣
320
1
T
.
ETHODS
)
HEORY
mV
1
⋅
) at the input to the feedback circuit. The
⎛
⎜ ⎜
⎝
R
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: [phone]
2
R
1
+
.
3
R
3
⎞
⎟ ⎟
⎠
−
V
ADJ
⋅
⎛
⎜ ⎜
⎝
R
R
The switch output
2
3
1
T H E O R Y O F O P E R A T I O N
⎞
⎟ ⎟
⎠
is calculated by
⎤
⎥
⎥
⎦
Integrated Products Division
SET
®
Microsemi
. The
eq. 2
eq. 1
I
P
LX1995-1) in the range of 10µH to 47µH due to saturation
of peak inductor current.
inductor current, the LX1995-x will extend the power
range. Smaller inductor values will reduce output voltage
ripple and are smaller in size.
O
inductor value (L), output capacitor value (C
switch current (I
(V
voltage ripple is a function of the output droop (as the
inductor current charges to I
error (i.e., typically 10mV), and the output overshoot
(energy stored in the inductor). When the switch is first
turn on, the total ripple voltage is:
of 0.5V of voltage drop across the inductor and FET
RDS
drop across the diode.
selected components, the output capacitor can then be
adjusted to meet the target ripple voltage requirements.
output voltage ripple is not a critical application
requirement.
∆
V
∆
NDUCTOR
ROGRAMMING
UTPUT
RIPPLE
V
V
Microsemi recommends the use inductors (for the
Output voltage ripple is depended on the selection of the
IN
The initial droop can be estimated with the assumption
The output overshoot calculated with 0.5V as voltage
Once the output voltage ripple is determined for the
The LX1995-x is targeted for LED driver applications;
DROOP
OVERSHOOT
) and the output voltage (V
ON
.
=
R
=
∆
IPPLE AND
V
P
⎛
⎜ ⎜
⎝
Miniature LED Driver
S
DROOP
C
RODUCTION
ELECTION AND
=
L
OUT
2
1
V
PEAK
⋅
IN
⎞
⋅ ⎟ ⎟
⎠
⎛
⎜ ⎜
⎝
+
V
C
−
∆
(
OUT
), load current (I
C
I
L
OUT
PK
0
V
APACITOR
5 .
OVERSHOOT
×
−
⎞
⋅ ⎟ ⎟
⎠
I
V
LED
D
(
IN
I
PEAK
ATA
PK
By increasing the average
)
+
O
−
0
UTPUT
), the feedback transition
OUT
5 .
I
S
+
S
LED
ELECTION
HEET
10mV
). The peak-to-peak
)
2
OUT
C
URRENT
), input voltage
OUT
LX1995
), peak
L
eq. 3
eq. 4
eq. 5
IMIT
Page 6