LX1911
TM
®
1.5A, 1.1MH
Z
PWM Step-Down Regulator
P
RODUCTION
D
ATA
S
HEET
DESCRIPTION
KEY FEATURES
Internal Reference 800mv ±2%
Accuracy (Line and Temp.)
4.0V to 6.0V Input Range
Internal Soft Start
Adj. Output From 0.8V to V
IN
Output Current (I >1.5A)
Quiescent Current < 550μA,
typical @ 23°C
1.1MHz PWM Frequency
Micro Lead-frame, Thin MO-
229, 6-Pin Package
APPLICATIONS/BENEFITS
Portable Microprocessor Core
Voltage Supplies
5V to 3V
The LX1911 operates as a Current
Mode PWM Buck regulator that
switches to PFM mode with light
loads. The entire regulator function
is implemented with minimal external
components.
The LX1911 responds quickly to
dynamic load changes using a high
bandwidth error amplifier and
internal compensation. Tight output
voltage regulation is maintained with
the compensated 800mV, ±2%
reference (line and temp regulation).
With two external resistors the output
voltage is easily programmed, from
800mV to 90% of V
IN
.
The regulator is capable of providing
an output load current of 1.5A and has
no minimum load current requirement
for stable operation. Current limit is
cycle-by-cycle to protect the switch.
Power conversion efficiency is
maximized with a low regulator IQ and
PFM mode of operation.
The LX1911 operational range
covers 4.0V to 6.0V, features include
power on delay; soft start to limit
inrush currents; and thermal shutdown
during fault conditions.
The 6-pin package is a small, RoHS
compliant / Pb-free, MO-229 with
excellent power dissipation capacity.
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OM
IMPORTANT:
For the most current data, consult
MICROSEMI’s
website:
http://www.microsemi.com
PRODUCT HIGHLIGHT
EFFICIENCY vs I
OUT
(V
IN
=5V)
100%
4.0 to 6V
4
6
Top V
O
=3.375V
Lower V
O
=1.25V
90%
80%
3.3µH
1.2V @ 1.5A
C
25K
20µF
10
C = 680p
50K
Ground
Range Tie-in
1
VIN
POWER
5
N.C.
3
VIN
ANALOG
4.7µF
2
SW
70%
60%
50%
40%
30%
1
10
100
mA
GND
FB
LX1911
1000
10000
Figure 1
– LX1911 Circuit Topology and Typical Efficiency Performance
PACKAGE ORDER INFO
T
A
(°C)
0 to 70
Input Voltage
4.5V – 5.5V
Output Voltage
Range
Adjustable
LX1911
LX1911
LD
Plastic MO-229
6 Pin 3mm²
LX1911CLD
RoHS Compliant / Pb-free
Part Markings
For Tape & Reel, append the letters “TR” to the part number. (i.e. LX1911CLD-TR)
Copyright
©
2004
Rev. 1.1, 2006-05-15
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 1
LX1911
TM
®
1.5A, 1.1MH
Z
PWM Step-Down Regulator
P
RODUCTION
D
ATA
S
HEET
ABSOLUTE MAXIMUM RATINGS
Input Voltage (IN) ..............................................................................................................-0.3V to 7.0V
SW to GND.............................................................................................................-0.3V to (V
IN
+ 0.3V)
V
FB
to GND..........................................................................................................................-0.3V to +2V
SW Peak Current (Internally Limited) ....................................................................................... 1800mA
Operating Temperature Range.......................................................................................-40°C to +125°C
Storage Temperature Range, T
A
...................................................................................... -65°C to 150°C
Maximum Junction Temperature.................................................................................................... 150°C
RoHS / Pb-free Package Peak Solder Reflow Temperature
(40 seconds maximum exposure) .......................................................................................260°C (+0,-5)
Note: Exceeding these ratings could cause damage to the device. All voltages are with respect to
Ground. Currents are positive into, negative out of specified terminal.
PACKAGE PIN OUT
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OM
FB
GND
SW
1
2
3
6
5
4
N.C.
VIN ANALOG
VIN PWR
LD P
ACKAGE
(Top View)
Note: Heatsink pad on bottom should be
connected to ground or left floating.
N.C. – No Connect
RoHS / Pb-free 100% Matte Tin Lead Finish
THERMAL DATA
LD
Plastic Micro Lead 6-Pin
THERMAL RESISTANCE
-
JUNCTION TO
A
MBIENT
,
θ
JA
25-40°C/W
†
Junction Temperature Calculation: T
J
= T
A
+ (P
D
x
θ
JA
).
The
θ
JA
numbers are guidelines for the thermal performance of the device/pc-board system. All of the
above assume no ambient airflow.
† PCB Layout Dependent
FUNCTIONAL PIN DESCRIPTION
N
AME
VIN ANALOG
VIN PWR
FB
GND
SW
D
ESCRIPTION
Unregulated supply voltage input, ranging from +4V to 6.0V for internal analog control circuitry.
Unregulated supply voltage input (+4V to 6.0V), high current path, connects to PMOS Source of PWM switch.
Feedback input for setting programming output voltage.
Circuit ground providing bias for IC operation and high frequency gate drive bias, can be connected to heatsink terminal.
Inductor and commutation diode connection point. Connects to internal PMOSFET source.
ELECTRICAL CHARACTERISTICS
Specifications apply over junction temperature of: 0
o
C < T
J
< 125
o
C for V
IN
= 5V (except where otherwise noted). Typical values are at T
A
=23°C.
Parameter
Operating Range
Feed Back Threshold
FB Input Current
Error Amplifier
Quiescent Operating Current
Soft Start, Vout Slew Rate
P-Channel Switch ON Resistance
Maximum Duty Cycle
SW Leakage Current
P-Channel Current Limit
PWM Frequency
PFM Mode Region
Feed Back PSRR
Closed Loop Load Regulation
Thermal Shutdown
Copyright
©
2004
Rev. 1.1, 2006-05-15
Symbol
V
IN
V
FBT
I
FB
BW
I
Q
(Pin 5)
Vo
R
DS(ON)
D
I
LEAK
I
LIM
F
OP-PWM
Io
Load Reg
T
SD
Test Conditions
Functional operation guaranteed by design
4.0V < V
IN
< 6.0V
V
FB
= 0.81V
Closed Loop
V
FB
> 0.825V, Rload Switch Pin < 1K ohms
Initial Power On or after Short Circuit
I
SW
= 1.0A
I
SW
= 1.0A (assured by design, not ATE tested)
V
FB
= 0.825V
Peak Current at Switch Pin (not dc current)
PWM Mode
PFM Mode
1hz < Frequency Vin < 10Khz
V
O
= 1.2V, 50mA < I
O
< 1.2A, ckt figure 1
(assured by design, not ATE tested)
Min
4.5
784
Typ
800
40
100
500
21
0.25
Max
5.5
816
75
850
50
0.5
5
1400
Units
V
mV
nA
Khz
µA
V/mS
Ω
E
LECTRICALS
E
LECTRICALS
80
1.6
700
100
0.01
1.9
1020
250
-40
0.85
%
µA
A
KHz
mA
dB
%V
O
°C
135
150
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 2
LX1911
TM
®
1.5A, 1.1MH
Z
PWM Step-Down Regulator
P
RODUCTION
D
ATA
S
HEET
SIMPLIFIED BLOCK DIAGRAM
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.C
OM
VIN
CL
Clock and
Ramp
PWM
S
R
SW
EA
BG Ref
800mV
GND
FB
Figure 2
– LX1911 Block Diagram
V
OUT
C
OUT
Inductor
Diode
C
OUT
C
OUT
V
IN
R
S
LX1911
VIN
ANALOG
GND
FB
A
PPLICATIONS
A
PPLICATIONS
C
S
VIN PWR
SW
N.C.
Rf
C
IN
C
OUT
Rf
GROUND
Figure 3
– PCB Layout Considerations
Copyright
©
2004
Rev. 1.1, 2006-05-15
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 3
LX1911
TM
®
1.5A, 1.1MH
Z
PWM Step-Down Regulator
P
RODUCTION
D
ATA
S
HEET
APPLICATION NOTE
F
UNCTIONAL
D
ESCRIPTION
The LX1911 is a Current Mode PWM regulator with
internal compensation.
The internal PMOS high side switch is protected with
current limit on a pulse by pulse basis and with thermal
shutdown. Thermal shutdown is activated with a junction
temperature of 160°C (typical) and has 20°C of hysteresis.
The regulator has an internal Power On Reset delay of
50-100µs to ensure all circuitry is operating before enabling
the Switch output.
Soft Start is activated upon initial power-on, or
following recovery from either thermal shutdown or short
circuit. The Soft start control block generates a voltage
ramp that clamps the error amplifier non-inverting
reference voltage. As this clamp voltage rises, the duty
cycle is gradually increased, thus limiting the peak inrush
currents.
PWM / PFM mode of operation is determined by the
load current condition. The PFM mode increases system
efficiency by reducing the switching frequency thus
switching losses. During light loading, I
OUT
< 200mA
typically, PFM mode becomes active, the switching
frequency begins to decrease, the frequency change occurs
over a continuous range, decreasing further as I
OUT
decreases.
O
UTPUT
V
OLTAGE
P
ROGRAMMING
Resistors R1 and R2 program the output voltage. The
value of R2 (lower resistor of divider) should be less than
10KΩ. The value of R1 can be determined using the
following equation, note V
REF
is also referred to as V
FBT
.
⎡⎛
V
⎞ ⎤
R1
=
R2
⎢⎜
OUT
⎟ −
1
⎥
⎜
⎟
⎢⎝
V
REF
⎠ ⎥
⎦
⎣
I
NDUCTOR
S
ELECTION
,
CONT
.
The internal compensation of the LX1911 design was
optimized for a 3.3µH inductor but will operate with an
inductor value in the range of 2.2µH and 22µH for a majority
of applications. The benefit of a larger inductor value can
increase efficiency at the lower output currents and reduces
output voltage ripple, thus output capacitance related to
ripple filtering. Smaller inductors typically provide smaller
package size (critical in many portable applications) at the
expense of increasing output ripple current. Regardless of
inductor value, selecting a device manufactured with a
ferrite-core produces lower losses at higher switching
frequencies and thus better overall performance. Larger
inductors may lead to diminished Step-Load response.
C
APACITOR
S
ELECTION
To minimize ripple voltage, output capacitors with a low
series resistance (ESR) are recommended. Multi-layer
ceramic capacitors with X5R or X7R dielectric make an
effective choice because they feature small size, very low
ESR, a temperature stable dielectric, and can be connected in
parallel to increase capacitance. Typical output capacitance
values of 10 to 30µF has proven effective. Other low ESR
capacitors such as solid tantalum, specialty polymer, or
organic semiconductor, make effective choices provided that
the capacitor is properly rated for the output voltage and
ripple current. Finally, choose an input capacitor of
sufficient size to effectively decouple the input voltage
source impedance (e.g., C
IN
> 4.7µF).
L
AYOUT
C
ONSIDERATIONS
The high peak currents and switching frequencies present in
DC/DC converter applications require careful attention to
device layout for optimal performance. Basic design rules
include: (1) maintaining wide traces for power components
(e.g., width > 50mils); (2) place C
IN
, C
OUT
, the Schottky
diode, and the inductor close to the LX1911; (3) minimizing
trace capacitance by reducing the etch area connecting the
SW pin to the inductor; and (4) minimizing the etch length to
the FB pin to reduce noise coupling into this high impedance
sense input. Other considerations include placing a 0.1uF
capacitor between the LX1911 V
OUT
pin and GND pin to
reduce high frequency noise and decoupling the VIN pin
using a 0.1µF capacitor. The LX1911 Switch has fast
switching speeds which may generate noise spikes when a
high capacitance Schottky diode is selected for the catch
diode. A simple snubber circuit, as shown in Figure 1, R=10
ohms and C=680pF has proven effective to reduce the spike
voltage generated at the Switch Pin / Diode connection.
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D
IODE
S
ELECTION
A Schottky diode is required for switching speed and low
forward voltage. Efficiency is determined mostly by the
diode’s forward voltage. The diode conducts 1-D%, for
V
OUT
= 1.2V this becomes 76% in a 5V system.
I
NDUCTOR
S
ELECTION
Selecting the appropriate inductor type and value ensures
optimal performance of the converter circuit for the
intended application. A primary consideration requires the
selection of an inductor that will not saturate at the peak
current level. EMI, output voltage ripple, and overall circuit
efficiency affect inductor choice. The inductor that works
best depends upon the application’s requirements and some
experimentation with actual devices in-circuit is typically
necessary to make the most effective choice.
Copyright
©
2004
Rev. 1.1, 2006-05-15
A
PPLICATIONS
A
PPLICATIONS
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 4
LX1911
TM
®
1.5A, 1.1MH
Z
PWM Step-Down Regulator
P
RODUCTION
D
ATA
S
HEET
CHARACTERISTIC CURVES
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Microsemi
.C
OM
CH2:VOUT
CH1:SWITCH
Figure 4–
Step Load Response 250ma-1.2A 3.3µH, 40µF
Blue: Vout 50mV/div AC; Green: I
STEP
200ma/div
Figure 5 –
Switching Waveforms: PFM Mode
Regulated VOUT vs Temperature
(Iout=10ma)
0.8%
0.6%
0.4%
0.2%
0.0%
-0.2%
-0.4%
-0.6%
-0.8%
-25
Figure 6 –
Power On and Soft Start
For I
OUT
10mA to 1amp
CH1:VIN
CH2:VOUT
0
25
50 75 100 125 150
G
RAPHS
G
RAPHS
TEMPERATURE, °C
Figure 7–
Vout Temperature Stability
Copyright
©
2004
Rev. 1.1, 2006-05-15
Microsemi
Integrated Products Division
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 5
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