LT3582 LINER [Linear Technology], LT3582 Datasheet - Page 14

• Manufacturer Part Number: LT3582
• Description: I2C Programmable Boost and Single Inductor Inverting DC/DC Converters with OTP
• Manufacturer: LINER [Linear Technology]

LT3582/LT3582-5/LT3582-12
APPLICATIONS INFORMATION
OTP2/REG2: Data in address 02h confi gures the output
voltage sequencing, sets a fi ne voltage adjust for V
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 (V
The LT3582 series contains two resistor dividers
which are programmable in the LT3582, to set the
output voltages. The positive output voltage V
is adjustable in 25mV steps by setting the V
REG0/OTP0 in addition to the V
where:
The V
the V
where:
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
V
put voltage, it will decay at a rate dependent on the load
current and capacitance. Confi guring a large increase in
magnitude of an output voltage can cause a large increase
in switch current to charge the output capacitor. Before
reconfi guring the outputs, consider forcing a soft-start
by asserting the SWOFF bit before writing the new V
V
soft-start sequence.
14
N
N
V
V
V
V
V
codes. Subsequently clearing SWOFF initiates the new
or V
P
PLUS
N
OUTP
OUTN
N
OUTN
= an integer value from 0 to 191
= an integer value from 0 to 255
bits in REG1/OTP1.
P
= 0 or 1
bits. When reducing the magnitude of an out-
= 3.2V + (V
= –1.2V – (V
voltage is adjustable in –50mV steps by setting
P
N
• 50mV) + (V
• 50mV)
P
PLUS
, V
PLUS
PLUS
bit in REG2/OTP2.
and V
• 25mV)
N
P
bits)
bits in
OUTP
OUTP
P
or
,
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 V
fi nal value. The regulator tries to charge the output capacitor
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 V
voltages ramp in proportion to their respective RAMP
voltages according to:
Proportionality Constant
RAMP pin ramp rate (V/Sec)
where:
For example, selecting I
V
(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 V
ramp up path is not followed B) inductor current ringing
occurs C) the V
V
OUTP
OUT
I
C
V
RAMP
RAMP
OUT
_RAMP _RATE =
= 12V results in a power-up ramp rate of 1.5Volt/ms
= RAMP pin charging current set by IRMP
= External RAMP pin capacitor (Farads)
= Output voltage during regulation
bits (1μA, 2μA, 4μA or 8μA for LT3582,
1μA for LT3582-5/LT3582-12)
OUTP
ramp rate is limited due to the output
RAMP
V
0.8V
OUT
= 1μA, C
•
C
I
OUT
RAMP
RAMP
RAMP
being far from its
Volts / Sec
= 10nF and
3582512f
OUTP
OUT