Datasheet go-LSP5503 Datasheet

LSP5503

3A Synchronous Step Down DC/DC Converter

 FEATURES

z 3A Output Current
z Wide 4.5V to 27V Operating Input Range

z Integrated 100mΩ Power MOSFET Switches

z Output Adjustable from 0.925V to 24V
z Up to 96% Efficiency
z Programmable Soft-Start
z Stable with Low ESR Ceramic Output Capacitors
z Fixed 400KHz Frequency
z Cycle-by-Cycle Over Current Protection
z Input Under Voltage Lockout
z SOP8L-EP Package

 TYPICAL APPLICATION

z Distributed Power Systems
z Networking Systems
z FPGA, DSP, ASIC Power Supplies
z Green Electronics/ Appliances
z Notebook Computers

 GENERAL DESCRIPTION

The LSP5503 is a monolithic synchronous buck
regulator. The device integrates 100m MOSFETS
that provide 3A continuous load cur-rent over a wide
operating input voltage of 4.5V to 27V. Current mode
control provides fast transient response and
cycle-by-cycle cur-rent limit.
An adjustable soft-start prevents inrush current at turn
on. In shutdown mode, the supply cur-rent drops
below 1µA.
This device, available in an SOP8L-EP package,
provides a very compact system solution with minimal
reliance on external components.

 PIN ASSIGNMENT

(Top View)

 PIN DESCRIPTION

Name No. Description

BS 1

IN 2

SW 3 Switch Output. Connect this pin to the switching end of the inductor.

G 4 Ground.

FB 5

COMP 6

EN 7

SS 8

Bootstrap. This pin acts as the positive rail for the high-side switch’s gate driver.
Connect a 0.1uF capacitor between BS and SW.
Input Supply. Bypass this pin to G with a low ESR capacitor. See Input Capacitor
in the Application Information section.

Feedback Input. The voltage at this pin is regulated to 0.925V. Connect to the
resistor divider between output and ground to set output voltage.
Compensation Pin. See Stability Compensation in the Application Information
section.
Enable Input. When higher than 2.5V, this pin turns the IC on. When lower than

1.3V, this pin turns the IC off. Output voltage is discharged when the IC is off. This
pin should not be left open.
Soft-Start Control Input. SS controls the soft-start period. Connect a capacitor from
SS to GND to set the soft-start period. A 0.1µF capacitor sets the soft-start period
to 15ms. To disable the soft-start feature, leave SS unconnected.

LSP5503 Rev. 1.3 1 of 10 2009/09

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LSP5503

3A Synchronous Step Down DC/DC Converter

 ABSOLUTE MAXIMUM RATINGS

Parameter Value Unit

IN Supply Voltage -0.3 to 30 V

SW Voltage -1 to VIN + 0.3 V

BS Voltage VSW – 0.3 to VSW + 6 V

EN, FB, COMP Voltage -0.3 to 6 V

Continuous SW Current Internally limited A

Junction to Ambient Thermal Resistance (JA)

(Test on Approximately 3 in

2

Copper Area 1OZ copper FR4 board)

Junction to Ambient Case Resistance (JC) 10 °C/W

Maximum Power Dissipation 1.8 W

Operating Junction Temperature -40 to 150 °C

Storage Temperature -55 to 150 °C

Lead Temperature (Soldering, 10 sec) 300 °C

(

Note: Exceeding these limits may damage the device. Exposure to absolute maximum rating conditions for long

periods may affect device reliability.)



ELECTRICAL CHARACTERISTICS

(V

= 12V, TA= 25°C unless otherwise specified.)

IN

Parameter Symbol Test Conditions Min. Typ. Max. Unit

Input Operating Voltage VIN V

Input Holdup Voltage V

= 1.0V, I

OUT

= 1.0V, I

OUT

= 0A to 3A 4.5 27 V

LOAD

= 0A to 3A 4.5 V

LOAD

Feedback Voltage VFB 4.5V  VIN  20V 0.900 0.925 0.950 V

Feedback Overvoltage Threshold 1.1 V

High-Side Switch-On Resistance 100

Low-Side Switch-On Resistance 100

High-Side Switch Leakage VEN = 0V, VSW = 0V 9 10 µA
Upper Switch Current Limit 3.5 4.0 A
Lower Switch Current Limit 0.9 A

COMP to Current Limit

Transconductance

Error Amplifier Transconductance GEA I

Error Amplifier DC Gain A

5.2 A/V

G

COMP

= ±10µA 800 µA/V

COMP

480 V/V

VEA

Switching Frequency fSW 350 400 470 kHz

Short Circuit Switching Frequency VFB = 0 150 kHz

Maximum Duty Cycle D

V

MAX

= 0.8V 90 %

FB

Minimum On Time 220 nS

EN Shutdown Threshold Voltage VEN Rising 1.1 1.3 1.5 V
EN Shutdown Threshold Voltage

Hys-terisis

200 mV

EN Lockout Threshold Voltage 2.2 2.5 2.7 V

EN Lockout Hysterisis 210 mV

Supply Current in Shutdown VEN = 0 0.3 3.0 µA

IC Supply Current in Operation VEN = 3V, VFB = 1.0V 1.4 1.5 mA

Input UVLO Threshold Rising UVLO VEN Rising 3.80 4.05 4.40 V

Input UVLO Threshold Hysteresis 210 mV

Soft-start Current VSS = 0V 12 µA

Soft-start Period CSS = 0.1µF 10 mS

Thermal Shutdown Temperature Hysteresis = 10°C 160 °C

50 °C/W

mΩ
mΩ

LSP5503 Rev. 1.3 2 of 10 2009/09

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LSP5503

3A Synchronous Step Down DC/DC Converter

 FUNCTIONAL BLOCK DIAGRAM

 FUNCTIONAL DESCRIPTION

The LSP5503 is a synchronous rectified, cur-rent-mode, step-down regulator. It regulates in-put voltages from 4.5V
to 23V down to an out-put voltage as low as 0.925V, and supplies up to 3A of load current.
The LSP5503 uses current-mode control to regulate the output voltage. The output voltage is measured at FB
through a resistive voltage divider and amplified through the internal trans-conductance error amplifier. The voltage
at the COMP pin is compared to the switch current
measured internally to control the output voltage.
The converter uses internal N-Channel MOSFET switches to step-down the input voltage to the regulated output
voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor
connected between SW and BS is needed to drive the high side gate. The boost capacitor is charged from the
internal 5V rail when SW is low.
When the LSP5503 FB pin exceeds 20% of the nominal regulation voltage of 0.925V, the over volt-age comparator
is tripped and the COMP pin and the SS pin are discharged to GND, forcing the high-side switch off.

LSP5503 Rev. 1.3 3 of 10 2009/09

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LSP5503

3A Synchronous Step Down DC/DC Converter

 APPLICATION INFORMATION

Output Voltage Setting

Figure1. Output Voltage Setting

Figure 1 shows the connections for setting the output voltage. Select the proper ratio of the two feedback resistors
RFB1 and RFB2 based on the output voltage. Typically, use RFB2  10k and determine RFB1 from the following
equation:

(1)

Inductor Selection

The inductor maintains a continuous current to the output load. This inductor current has a ripple that is dependent
on the inductance value: higher inductance reduces the peak-to-peak ripple current. The trade off for high
inductance value is the increase in inductor core size and series resistance, and the reduction in current handling
capability. In general, select an inductance value L based on the ripple current requirement:

)VV(V

L−•=

where V

is the input voltage, V

IN

output current, and K
ripple current being 30% of the maximum output current.
With this inductor value, the peak inductor current is I
is less that the 4A current limit. Finally, select the inductor core size so that it does not saturate at 4A. Typical
inductor values for various output voltages are shown in Table 1.

Input Capacitor

The input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the
converter. A low ESR capacitor is highly recommended. Since large current flows in and out of this capacitor during
switching, its ESR also affects efficiency.
The input capacitance needs to be higher than 10µF. The best choice is the ceramic type; however, low ESR
tantalum or electrolytic types may also be used provided that the RMS ripple current rating is higher than 50% of the
output current. The input capacitor should be placed close to the IN and G pins of the IC, with the shortest traces
possible. In the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1µF ceramic
capacitor is placed right next to the IC.

KIfV

OUTINOUT

RIPPLEOUT M AXSWIN

(2)

is the output voltage, fSW is the switching frequency, I

OUT

is the ripple factor. Typically, choose K

RIPPLE

OUT

V

1.0V 1.2V 1.5V 1.8V 2.5V 3.3V 5V

OUT

L 4.7uH 4.7uH 4.7uH 4.7uH 6.8µH 6.8µH 10µH

Table 1. Typical Inductor Values

• (1 + K

is the maximum

OUTMAX

= 30% to correspond to the peak-to-peak

RIPPLE

/ 2). Make sure that this peak inductor current

RIPPLE

LSP5503 Rev. 1.3 4 of 10 2009/09

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LSP5503

3A Synchronous Step Down DC/DC Converter

Output Capacitor

The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is:

RKIV =

ESRRIPPLEOUTMAXRIPPLE

where I

is the maximum output current, K

OUTMAX

is the switching frequency, L is the inductor value, and C

+

capacitors, R

is very small and does not contribute to the ripple. Therefore, a lower capacitance value can be

ESR

used for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is dominated by R
multiplied by the ripple current. In that case, the output capacitor is chosen to have sufficiently low ESR.

For ceramic output capacitors, typically choose a capacitance of about 22µF. For tantalum or electrolytic capacitors,
choose a capacitor with less than 50m ESR.

Optional Schottky Diode

During the transition between high-side switch and low-side switch, the body diode of the low side power MOSFET
conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be
paralleled between the SW pin and GND pin to improve overall efficiency. Table 2 lists example Schottky diodes and

their Manufacturers.

Stability Compensation

C

The feedback loop of the IC is stabilized by the components at the COMP pin, as shown in Figure 2. The DC loop
gain of the system is determined by the following equation:

IN

2

LCf28V•

SW

is needed only for high ESR output capacitor

COMP2

OUT

(3)

is the ripple factor, R

RIPPLE

is the output capacitance. In the case of ceramic output

OUT

Figure 2. Stability Compensation

is the ESR of the output capacitor, fSW

ESR

ESR

(4)

The dominant pole P1 is due to C

G

f =

1P

EA

CAπ2

COMPVEA

(5)

COMP

:

The second pole P2 is the output pole:

I

f =

2P

The first zero Z1 is due to R

f =

1Z

And finally, the third pole is due to R

f =

3P

LSP5503 Rev. 1.3 5 of 10 2009/09

OUT

CVπ2

OUTOUT

(6)

COMP

and C

COMP

:

1

CRπ2

COMPCOMP

(7)

COMP

and C

COMP2

(if C

COMP2

is used):

1

CRπ2

2COMPCOMP

(8)

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LSP5503

3A Synchronous Step Down DC/DC Converter

The following steps should be used to compensate the IC:
STEP1. Set the crossover frequency at 1/10 of the switching frequency via RCOMP:

(9)

but limit RCOMP to 10k maximum.
STEP2. Set the zero fZ1 at 1/4 of the crossover frequency. If RCOMP is less than 10k, the equation for CCOMP is:

5

−

×

COMP

=

C

If RCOMP is limited to 10k, then the actual crossover frequency is 10/ (VOUTCOUT). Therefore:

COMP

STEP3. If the output capacitor’s ESR is high enough to cause a zero at lower than 4 times the crossover frequency,
an additional compensation capacitor CCOMP2 is required. The condition for using CCOMP2 is:

R

ESRCOUT

R

≥

108.1

COMP

×=

Min

)F(

(10)

5

−

⎛
⎜
⎜
⎝

OUTOUT

6

−

101.1

×

C

OUT

)F(CV102.1C

(11)

⎞

OUT

⎟
⎟
⎠

•

Ω

)(V012.0,

(12)

And the proper value for C

RC

C =

2COMP

Though C

COMP2

ESRCOUTOUT

R

COMP

is unnecessary when the output capacitor has sufficiently low ESR, a small value C

is:

COMP2

(13)

100pF may improve stability against PCB layout parasitic effects.
Table 3 shows some calculated results based on the compensation method above.

VOUT COUT RCOMP CCOMP CCOMP2

1.0V 22µF Ceramic

1.2V 22µF Ceramic

1.8V 22µF Ceramic

2.5V 22µF Ceramic

3.3V 22µF Ceramic

5V 22µF Ceramic

1.0V 47µF SP Cap

1.2V 47µF SP Cap

1.8V 47µF SP Cap

2.5V 47µF SP Cap

3.3V 47µF SP Cap

5V 47µF SP Cap

1.0V

1.2V

1.8V

2.5V

3.3V

5V

470µF/6.3V/30mΩ 10kΩ
470µF/6.3V/30mΩ 10kΩ
470µF/6.3V/30mΩ 10kΩ
470µF/6.3V/30mΩ 10kΩ
470µF/6.3V/30mΩ 10kΩ

470µF/10V/30mΩ 10kΩ

1.5kΩ

1.7kΩ

2.2kΩ

3.6kΩ

4.7kΩ

6.8kΩ

3.0kΩ

3.6kΩ

5.6kΩ

6.8kΩ
10kΩ
10kΩ

10nF 100pF

10nF 100pF

6.8nF 100pF

4.7nF 100pF

3.3nF 47pF

2.2nF 47pF

6.8nF 470pF

4.7nF 330pF

3.3nF 220pF

2.2nF 200pF

2.0nF 150pF

2.2nF 150pF

2.2nF 1nF

3.3nF 1nF

4.7nF 1nF

6.8nF 1nF

8.2nF 1nF

10nF 1nF

Table3. Typical Compensation for Different Output Voltages and Output Capacitors

COMP2

such as

LSP5503 Rev. 1.3 6 of 10 2009/09

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LSP5503

3A Synchronous Step Down DC/DC Converter

Figure 3 shows a sample LSP5503 application circuit generating 5V/3A output.

(Option)

Figure3. LSP5503 5V/3A Output Application

Figure 4 shows a sample LSP5503 application circuit generating 1.0V/3A output.

(Option)

Figure4. LSP5503 1.0V/3A Output Application

LSP5503 Rev. 1.3 7 of 10 2009/09

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LSP5503

3A Synchronous Step Down DC/DC Converter

 TYPICAL CHARACTERISTICS

Start up soft start Vin=12V, Vout=5V Iout=3A Operating status Vin=12V, Vout=5V Iout=3A

ripple of Vout Vin=12V, Vout=5V Iout=3A SCP

12Vin 5.0Vout Efficiency curve 12Vin 1.0Vout Efficiency curve

100

90

Efficiency vs Input Voltage（Vout=5.0V)

VIN=8V
VIN=12V
VIN=18V

=

Efficiency vs Input Voltage（Vout=1.0V)

100

90

VIN=5V VIN=8V
VIN=12V VIN=18V
VIN=23V

80

η(%)

70

80

η(%)

70

60

60

50

0 500 1000 1500 2000 2500 3000

Io(mA)

50

0 500 1000 1500 2000 2500 3000

LSP5503 Rev. 1.3 8 of 10 2009/09

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Io(mA)

LSP5503

3A Synchronous Step Down DC/DC Converter

 ORDERING INFORMATION

 MARKING INFORMATION

LSP5503 Rev. 1.3 9 of 10 2009/09

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LSP5503

3A Synchronous Step Down DC/DC Converter

 PACKAGE INFORMATION

E1

D1



7

(4х)

D

e

B

Symbol

Dimensions In Millimeters Dimensions In Inches

Min. Nom. Max. Min. Nom. Max.

A 1.35 1.60 1.75 0.053 0.063 0.069
A1 0.05 0.15 0..002 0.006
A2 1.35 1.45 1.55 0.053 0.057 0.061

B 0.33 0.41 0.51 0.013 0.016 0.020

C 0.19 0.20 0.25 0.0075 0.008 0.010

D 4.70 4.90 5.10 0.185 0.196 0.200
D1 3.202 3.402 0.126 0.134

E 3.80 3.90 4.00 0.148 0.154 0.160
E1 2.313 2.513 0.091 0.099

e 1.27TYP. 0.050TYP.

H 5.80 5.99 6.30 0.228 0.236 0.248

L 0.38 0.71 1.27 0.015 0.028 0.050
 0 8 0

H

E

θ



7

(4х)



8



A2

A1

0.015х45

A

VIEW “A”



LSP5503 Rev. 1.3 10 of 10 2009/09

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