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LT3582EUD

I2C Programmable Boost and Single Inductor Inverting DC/DC Converters with OTP

Linear Technology 

LT3582/LT3582-5/LT3582-12

APPLICATIONS INFORMATION

OTP2/REG2: Data in address 02h configures the output

voltage sequencing, sets a fine voltage adjust for VOUTP,

and determines if further OTP programming is permitted or

not. Proper uses of the bits in address 02h are discussed

in the following sections.

Setting the Output Voltages (VP, VPLUS and VN bits)

The LT3582 series contains two resistor dividers

which are programmable in the LT3582, to set the

output voltages. The positive output voltage VOUTP

is adjustable in 25mV steps by setting the VP bits in

REG0/OTP0 in addition to the VPLUS bit in REG2/OTP2.

VOUTP = 3.2V + (VP • 50mV) + (VPLUS • 25mV)

where:

VP = an integer value from 0 to 191

VPLUS = 0 or 1

The VOUTN voltage is adjustable in –50mV steps by setting

the VN bits in REG1/OTP1.

VOUTN = –1.2V – (VN • 50mV)

where:

VN = an integer value from 0 to 255

Dynamically Changing the Output Voltage (LT3582 Only):

After output regulation has been reached, it’s possible to

change the output voltages by writing new values to the

VN or VP bits. When reducing the magnitude of an out-

put voltage, it will decay at a rate dependent on the load

current and capacitance. Configuring a large increase in

magnitude of an output voltage can cause a large increase

in switch current to charge the output capacitor. Before

reconfiguring the outputs, consider forcing a soft-start

by asserting the SWOFF bit before writing the new VP or

VN codes. Subsequently clearing SWOFF initiates the new

soft-start sequence.

Soft-Start/Output Voltage Ramping (IRMP bits)

The LT3582 series contains soft-start circuitry to control

the output voltage ramp rates, therefore limiting peak

switch currents during start-up. High switch currents are

inherent in switching regulators during startup since the

feedback loop is saturated due to VOUT being far from its

final value.The regulator tries to charge the outputcapacitor

as quickly as possible which results in large currents.

Capacitors must be connected from RAMPP & RAMPN

to ground for soft-start. During shutdown or when the

SWOFF bit is set, the RAMP capacitors are discharged

to ground. After SHDN rises or SWOFF is cleared, the

capacitors are charged by programmable (LT3582 only)

currents, thus creating linear voltage ramps. The VOUT

voltages ramp in proportion to their respective RAMP

voltages according to:

VOUT

_

RAMP

_

RATE

=

0VO.8UVT





•

CIRRAAMMPP







Volts

/

Sec

Proportionality Constant

RAMP pin ramp rate (V/Sec)

where:

IRAMP = RAMP pin charging current set by IRMP

bits (1μA, 2μA, 4μA or 8μA for LT3582,

1μA for LT3582-5/LT3582-12)

CRAMP = External RAMP pin capacitor (Farads)

VOUT = Output voltage during regulation

For example, selecting IRAMP = 1μA, CRAMP = 10nF and

VOUTP = 12V results in a power-up ramp rate of 1.5Volt/ms

(see Figure 6).

Ramp rates less than 1-10V/ms generally result in good

startup characteristics. The outputs should linearly follow

the RAMPx voltages with no distortions. Figure 7 shows

an excessive startup ramp rate of ~120V/ms in which sev-

eral startup issues have occurred: A) the expected VOUTP

ramp up path is not followed B) inductor current ringing

occurs C) the VOUTP ramp rate is limited due to the output

14

3582512f

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