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LTC4006_1中文资料

LTC4006

TEST PERFORMED ON DEMOBOARD

V IN = 15VDC

CHARGER = ON I CHARGE = <10mA V s OF PFET (5V/DIV)

I d (REVERSE) OF PFET (5A/DIV)

V gs OF PFET (2V/DIV)

4006 G01

LTC4006-2INFET = 1/2 Si4925DY 1.25μs/DIV

4006 G04

LOAD CURRENT = 1A, 2A, 3A DCIN = 20V LTC4006-2

DISCONNECT

RECONNECT

1A STEP

3A STEP 3A STEP 1A STEP

–

LTC4006

4006fa

Soft-Start

The LTC4006 is soft started by the 0.12μF capacitor on the I TH pin. On start-up, I TH pin voltage will rise quickly to 0.5V,then ramp up at a rate set by the internal 40μA pull-up current and the external capacitor. Battery charging current starts ramping up when I TH voltage reaches 0.8V and full current is achieved with I TH at 2V. With a 0.12μF capacitor, time to reach full charge current is about 2ms and it is assumed that input voltage to the charger will reach full value in less than 2ms. The capacitor can be increased up to 1μF if longer input start-up times are needed.

Input and Output Capacitors

The input capacitor (C2) is assumed to absorb all input switching ripple current in the converter, so it must have adequate ripple current rating. Worst-case RMS ripple current will be equal to one half of output charging current.Actual capacitance value is not critical. Solid tantalum low ESR capacitors have high ripple current rating in a rela-tively small surface mount package, but caution must be used when tantalum capacitors are used for input or output bypass . High input surge currents can be created when the adapter is hot-plugged to the charger or when a battery is connected to the charger. Solid tantalum capaci-tors have a known failure mechanism when subjected to very high turn-on surge currents. Only Kemet T495 series of “Surge Robust” low ESR tantalums are rated for high surge conditions such as battery to ground.

The relatively high ESR of an aluminum electrolytic for C1,located at the AC adapter input terminal, is helpful in reducing ringing during the hot-plug event. Refer to Appli-cation Note 88 for more information.

Highest possible voltage rating on the capacitor will mini-mize problems. Consult with the manufacturer before use.Alternatives include new high capacity ceramic (at least 20μF) from Tokin, United Chemi-Con/Marcon, et al. Other alternative capacitors include OS-CON capacitors from Sanyo.

The output capacitor (C3) is also assumed to absorb output switching current ripple. The general formula for capacitor current is:

APPLICATIO S I FOR ATIO

W U

U I V V

V L f RMS

BAT BAT DCIN =

()?????

()()02911.–For example:

V DCIN = 19V, V BAT = 12.6V, L1 = 10μH, and f = 300kHz, I RMS = 0.41A.

EMI considerations usually make it desirable to minimize ripple current in the battery leads, and beads or inductors may be added to increase battery impedance at the 300kHz switching frequency. Switching ripple current splits be-tween the battery and the output capacitor depending on the ESR of the output capacitor and the battery impedance.If the ESR of C3 is 0.2? and the battery impedance is raised to 4? with a bead or inductor, only 5% of the current ripple will flow in the battery.Inductor Selection

Higher operating frequencies allow the use of smaller inductor and capacitor values. A higher frequency gener-ally results in lower efficiency because of MOSFET gate charge losses. In addition, the effect of inductor value on ripple current and low current operation must also be considered. The inductor ripple current ?I L decreases with higher frequency and increases with higher V IN .

()()?????

?I f L V V V L OUT OUT IN 1

1–Accepting larger values of ?I L allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is ?I L = 0.4(I MAX ). In no case should ?I L exceed 0.6(I MAX ) due to limits imposed by I REV and CA1. Remember the maximum ?I L occurs at the maxi-mum input voltage. In practice 10μH is the lowest value recommended for use.

Lower charger currents generally call for larger inductor values. Use Table 4 as a guide for selecting the correct inductor value for your application.

LTC4006

4006fa

Table 4

MAXIMUM

INPUT MINIMUM INDUCTOR

AVERAGE CURRENT (A)

VOLTAGE (V)

VALUE (μH)

1≤2040 ±20%1>2056 ±20%2≤2020 ±20%2>2030 ±20%3≤2015 ±20%3>2020 ±20%4≤2010 ±20%4

>20

15 ±20%

Charger Switching Power MOSFET and Diode Selection

Two external power MOSFETs must be selected for use with the charger: a P-channel MOSFET for the top (main)switch and an N-channel MOSFET for the bottom (syn-chronous) switch.

The peak-to-peak gate drive levels are set internally. This voltage is typically 6V. Consequently, logic-level threshold MOSFETs must be used. Pay close attention to the BV DSS specification for the MOSFETs as well; many of the logic level MOSFETs are limited to 30V or less.

Selection criteria for the power MOSFETs include the “ON”resistance R DS(ON), total gate capacitance Q G , reverse transfer capacitance C RSS , input voltage and maximum output current. The charger is operating in continuous mode at moderate to high currents so the duty cycles for the top and bottom MOSFETs are given by:Main Switch Duty Cycle = V OUT /V IN

Synchronous Switch Duty Cycle = (V IN – V OUT )/V IN .The MOSFET power dissipations at maximum output current are given by:

PMAIN =V OUT /V IN (I 2MAX )(1 + δ?T)R DS(ON)

+ k(V 2IN )(I MAX )(C RSS )(f OSC )PSYNC =(V IN – V OUT )/V IN (I 2MAX )(1 + δ?T)R DS(ON)Where δ is the temperature dependency of R DS(ON) and k is a constant inversely related to the gate drive current.Both MOSFETs have I 2R losses while the PMAIN equation includes an additional term for transition losses, which are

highest at high input voltages. For V IN < 20V the high current efficiency generally improves with larger MOSFETs,while for V IN > 20V the transition losses rapidly increase to the point that the use of a higher R DS(ON) device with lower C RSS actually provides higher efficiency. The syn-chronous MOSFET losses are greatest at high input volt-age or during a short circuit when the duty cycle in this switch is nearly 100%. The term (1 + δ?T) is generally given for a MOSFET in the form of a normalized R DS(ON) vs temperature curve, but δ = 0.005/°C can be used as an approximation for low voltage MOSFETs. C RSS is usually specified in the MOSFET characteristics; if not, then C RSS can be calculated using C RSS = Q GD /?V DS . The constant k =2 can be used to estimate the contributions of the two terms in the main switch dissipation equation.If the charger is to operate in low dropout mode or with a high duty cycle greater than 85%, then the topside P-channel efficiency generally improves with a larger MOSFET. Using asymmetrical MOSFETs may achieve cost savings or efficiency gains.

The Schottky diode D1, shown in the Typical Application on the back page, conducts during the dead-time between the conduction of the two power MOSFETs. This prevents the body diode of the bottom MOSFET from turning on and storing charge during the dead-time, which could cost as much as 1% in efficiency. A 1A Schottky is generally a good size for 4A regulators due to the relatively small average current. Larger diodes can result in additional transition losses due to their larger junction capacitance.The diode may be omitted if the efficiency loss can be tolerated.

Calculating IC Power Dissipation

The power dissipation of the LTC4006 is dependent upon the gate charge of the top and bottom MOSFETs (Q G1 and Q G2 respectively). The gate charge is determined from the manufacturer’s data sheet and is dependent upon both the gate voltage swing and the drain voltage swing of the MOSFET. Use 6V for the gate voltage swing and V DCIN for the drain voltage swing.

P D = V DCIN ? (f OSC (Q G1 + Q G2) + I DCIN )

APPLICATIO S I FOR ATIO

W U

LTC4006

Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.19

LTC4006

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LT 0506 REV A ? PRINTED IN USA