LM3200TLX datasheet(13/15 Pages) NSC

13 / 15 page

Current Limiting (Continued)

fore, the device may not startup if an excessive load is

connected to the output when the device is enabled. The

synchronous rectifier is off in the timed current limit mode.

Timed current limit prevents the loss of current control seen

in some products when the output voltage is pulled low in

serious overload conditions.

A current limit is also provided for the NFET. This is approxi-

mately −500 mA. Both the NFET and the PFET are turned off

in negative current limit until the PFET is turned on again at

the beginning of the next cycle. The negative current limit

inhibits buildup of excessive inductor current. In the Bypass

mode, the bypass current limit is 1000 mA(typ). The output

voltage drops when the bypass current limit kicks in.

Thermal Overload Protection

The LM3200 has a thermal overload protection function that

operates to protect the device from short-term misuse and

overload conditions. When the junction temperature exceeds

around 150˚C, the device inhibits operation. Both the PFET

and the NFET are turned off in PWM mode, and the Bypass

PFET is turned off in Bypass mode. When the temperature

drops below 130˚C, normal operation resumes. Prolonged

operation in thermal overload conditions may damage the

device and is considered bad practice.

Application Information

SETTING THE OUTPUT VOLTAGE

The LM3200 features a pin-controlled variable output volt-

age to eliminate the need for external feedback resistors. It

can be programmed for an output voltage from 0.8V to 3.6V

by setting the voltage on the V

CON pin, as in the following

formula:

OUT =3xVCON

When V

CON is between 0.267V and 1.20V, the output volt-

age will follow proportionally by 3 times of V

CON.

If V

CON is over 1.20V (VOUT = 3.6V), sub-harmonic oscilla-

tion may occur because of insufficient slope compensation.

If V

CON voltage is less than 0.267V (VOUT = 0.8V), the output

voltage may not be regulated due to the required on-time

being less than the minimum on-time (50ns). The output

voltage can go lower than 0.8V providing a limited V

IN range

is used. Refer to datasheet curve (Low V

CON Voltage vs

Output Voltage) for details. This curve is for a typical part and

there could be part to part variation for output voltages less

than 0.8V over the limited V

IN range. In addition, if VCON is

less than approx. 0.15V, the LM3200 output is turned off, but

the internal bias circuits are still active.

INDUCTOR SELECTION

A 2.2 µH inductor with saturation current rating over 940 mA

is recommended for almost all applications. The inductor

resistance should be less than 0.3

Ω for better efficiency.

Table 1 lists suggested inductors and suppliers.

TABLE 1. Suggested Inductors and Their Suppliers

Model

Size (WxLxH) [mm]

Vendor

DO3314-222MX

3.3 x 3.3 x 1.4

Coilcraft

VLF3010AT-2R2M1R0 2.6 x 2.8 x 1.0

TDK

MIPW3226D2R2M

3.2 x 2.6 x 1.0

FDK

Model

Size (WxLxH) [mm]

Vendor

LPO3310-222MX

3.3 x 3.3 x 1.0

Coilcraft

If a higher value inductor is used the LM3200 may become

unstable and exhibit large under or over shoot during line,

load and V

CON transients. If smaller inductance value is

used, slope compensation maybe insufficient causing sub-

harmonic oscillations. The device has been tested with in-

ductor values in the range 1.55µH to 3.1µH to account for

inductor tolerances.

For low-cost applications, an unshielded bobbin inductor can

be used. For noise-critical applications, an unshielded or

shielded-bobbin inductor should be used. A good practice is

to layout the board with footprints accommodating both

types for design flexibility. This allows substitution of an

unshielded inductor, in the event that noise from low-cost

bobbin models is unacceptable. Saturation occurs when the

magnetic flux density from current through the windings of

the inductor exceeds what the inductor’s core material can

support with a corresponding magnetic field. This can cause

poor efficiency, regulation errors or stress to a DC-DC con-

verter like the LM3200.

CAPACITOR SELECTION

The LM3200 is designed to be used with ceramic capacitors.

Use a 10 µF ceramic capacitor for the input and a 4.7 µF

ceramic capacitor for the output. Ceramic capacitors such as

X5R, X7R and B are recommended for both filters. These

provide an optimal balance between small size, cost, reliabil-

ity and performance for cell phones and similar applications.

Table 2 lists suggested capacitors and suppliers.

TABLE 2. Suggested Capacitors and Their Suppliers

Model

Vendor

LMK212BJ475MG

Taiyo-Yuden

C2012X5R1A475K

TDK

GRM188R61A475K

Murata

C3216X5R1A106K

TDK

The DC bias characteristics of the capacitor must be consid-

ered when making the selection. If smaller case size such as

0603 is selected, the dc bias could reduce the cap value by

as much as 40%, in addition to the 20% tolerances and 15%

temperature coefficients. Request dc bias curves from

manufacturer when making selection.The device has been

designed to be stable with output capacitors as low as 3 µF

to account for capacitor tolerances.This value includes dc

bias reduction, manufacturing tolerences and temp coeffi-

cients.

The input filter capacitor supplies AC current drawn by the

PFET switch of the LM3200 in the first part of each cycle and

reduces the voltage ripple imposed on the input power

source. The output filter capacitor absorbs the AC inductor

current, helps maintain a steady output voltage during tran-

sient load changes and reduces output voltage ripple. These

capacitors must be selected with sufficient capacitance and

sufficiently low ESR (Equivalent Series Resistance) to per-

form these functions. The ESR of the filter capacitors is

generally a major factor in voltage ripple.

MICRO SMD PACKAGE ASSEMBLY AND USE

Use of the Micro SMD package requires specialized board

layout, precision mounting and careful re-flow techniques, as

detailed in National Semiconductor Application Note 1112.

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