LT3587 데이터 시트보기 (PDF) - Linear Technology

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LT3587

High Voltage Monolithic Inverter and Dual Boost

Linear Technology 

LT3587

APPLICATIONS INFORMATION

capacitance will slow down the switching waveform, which

can cause significant switching losses at 1MHz switch-

ing frequency. Some recommended Schottky diodes are

listed in Table 2.

Table 2. Recommended Schottky Diodes

PART

NUMBER

DIODE

FORWARD FORWARD CAPACI-

CURRENT VOLTAGE TANCE

(mA) DROP (V) (pF at 10V) MANUFACTURER

RSX051VA-30 1000

0.35

30

ROHM

www.rohm.com

PMEG401OCEJ 500

0.49

PMEG2005EB

500

0.43

25

NXP/Phillips

8

www.nxp.com

IR05H40CSPTR 500

0.48

39

Vishay

www.vishay.com

B0540WS

500

0.48

20

Diodes Inc.

www.diodes.com

ZLLS400

520

0.53

17

Zetex

www.zetex.com

Smaller Footprint Inverter Topology

In certain applications with higher tolerance of current ripple

at the output of the inverter, the inductor L3 can be replaced

with a Schottky diode. Since the Schottky diode footprint

is usually smaller than the inductor footprint, this alternate

topology is recommended if a smaller overall solution is a

must. Note that this topology is only viable if the absolute

value of the inverter output is greater than VIN.

This Schottky diode is configured with the anode connected

to the output of the inverter and the cathode to the output

end of the flying capacitor C2 as shown in Figure 2.

VVIN

2.5V TO 4.5V

LT3587

RFB1

1M

FB2

SW2

L2

15μH

3587 F02

C2

2.2μF

D3

INVERTER

OUTPUT

–8V, 100mA

DS2

C7

22μF

Figure 2. Inverter Configured with a Schottky

Diode in Place of the Output Inductor

The same constraints as the other Schottky diodes ap-

ply for selecting D3. Therefore, the same recommended

Schottky diodes in Table 2 can be used for D3.

Boost3 Overcurrent and Overvoltage Protection

As briefly discussed in the Operation section, the regula-

tion loop of Boost3 uses the maximum of the two voltages

at VFB3 and IFB3 as feedback information to set the peak

current of its power switch Q3. This allows for the Boost3

loop to be configured as either a boost voltage regulator

or a boost current regulator (Figure 3). Furthermore, this

architecture also allows for a programmable current limit on

voltage regulation or voltage limit on current regulation.

VVIN

VVIN

VIN

SW3

VIN

SW3

CAP3

LT3587

BOOST3

VOLTAGE

VOUT3

REGULATOR

VFB3

EN/SS3

IFB3

RVFB3

VOLTAGE

REGULATION

FEEDBACK

RIFB3 RESISTOR

CAP3

LT3587

BOOST3

CURRENT

VOUT3

REGULATOR

VFB3

EN/SS3

IFB3

RVFB3

OPTIONAL

PROGRAMMABLE

VOLTAGE LIMIT

RIFB3 RESISTOR

OPTIONALPROGRAMMABLE

CURRENT LIMIT RESISTOR

CURRENT REGULATION

FEEDBACK RESISTOR

3587 F03

Figure 3. Boost3 Configured as a Voltage

Regulator and as a Current Regulator

When configured as a boost voltage regulator, a feedback

resistor from the output pin VOUT3 to the VFB3 pin sets the

voltage level at VOUT3 at a fixed level. In this case, the IFB3

pin can either be grounded if no current limiting is desired

or connected to ground with a resistor such that:

ILIMIT = 200 • (0.8V/RIFB3)

where ILIMIT is the desired output current limit value. Recall

that the pull-up current on the IFB3 pin is controlled to be

typically 1/200 of the output load current at the VOUT3

pin. In this case, when the load current is less than ILIMIT,

the Boost3 loop regulates the voltage at the VFB3 pin to

0.8V. When there is an increase in load current beyond

ILIMIT, the voltage at VFB3 starts to drop and the voltage

at IFB3 rises above 0.8V. The Boost3 loop then regulates

the voltage at the IFB3 pin to 0.8V, limiting the output

3587fc

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