LTC3589 Linear Technology, LTC3589 Datasheet - Page 18

• Manufacturer Part Number: LTC3589
• Description: 8-Output Regulator
• Manufacturer: Linear Technology

LTC3589
OPERATION
LDO Regulator 4
LDO4 has four output voltage options that are controlled
by the contents of command register L2DTV2 bits 6
and 5. By default, pin EN_LDO34 enables and disables
LDO3 and LDO4 simultaneously when command register
bits OVEN[6] and OVEN[7] are LOW. When EN_LDO34 is
LOW, LDO3 and LDO4 are controlled by writing to com-
mand register bits OVEN[6] and OVEN[7] respectively.
When command register bit L2DTV2[7] is HIGH, control of
LDO4 is disconnected from pin EN_LDO34 and controlled
by command register bit OVEN[7] regardless of the status
of EN_LDO34. Table 3 shows the I
settings used to control LDO4.
Table 3. LDO 4 Command Register Settings
COMMAND
REGISTER[BIT]
OVEN[6]
SCR2[6]
L2DTV2[6-5]
L2DTV2[7]
* Denotes Default Power-On Value
STEP-DOWN SWITCHING REGULATORS
Output Voltage Programming
Each of the step-down converters uses a dynamically
slewing DAC output for its reference. The full-scale output
voltage is set by using a resistor divider connected from
the step-down switching regulator output to the feedback
pins (B1_FB, B2_FB, and B3_FB) as shown in Figure 3.
Set the output voltage of step-down switching regulators
using the following formula:
BxDTVx is the decimal value of the fi ve bit binary number
in the I
and BxDTV2 default to 11001 to output a reference voltage
18
V
OUT
2
C BxDTV1 or BxDTV2 command registers. BxDTV1
= 1+
VALUE
R2
R1
00*
01
10
11
0
1
0*
1
0*
1
• (0.3625 + BxDTVx • 0.0125)(V)
SETTING
Disable
Enable
Wait for Output Below 300mV Before Enable
Enable Immediately
V
V
V
V
LDO4 Enable Controlled by EN_LDO34
LDO4 Enable Controlled by OVEN[6]
LDO4
LDO4
LDO4
LDO4
= 2.8V
= 2.5V
= 1.8V
= 3.3V
2
C command register
of 0.675V. Typical values for R1 are in the range of 40k
to 1M. The capacitor C
feedback resistors and the input capacitance on the FB pin
and also helps to improve load step transient response.
A value of 10pF is recommended for most applications.
Experimentation with capacitor sizes between 10pF and
33pF may yield improved transient response.
Operating Modes
The step-down switching regulators include three possible
operating modes to meet the noise and power needs of a
variety of applications.
In pulse-skipping mode, at the start of every cycle, a latch
is set that turns on the main P-channel MOSFET switch.
During the cycle, a current comparator compares the
peak inductor current to the output of an error amplifi er.
The output of the current comparator resets the latch.
At this time the P-channel MOSFET switch turns off and
the N-channel MOSFET synchronous rectifi er turns on.
The N-channel MOSFET synchronous rectifi er will turn
off when the end of the clock cycle is reached or if the
inductor current drops through zero. Using this method
of operation, the error amplifi er adjusts the peak inductor
current to deliver the required output power. All necessary
loop compensation is internal to the step-down switching
regulator requiring only a single ceramic output capacitor
for stability. At light loads in pulse-skipping mode, the
inductor current may reach zero on each pulse that will
EN
MODE
5
Figure 3. Step-Down Switching Regulator
Application Circuit
CONTROL
PWM
DAC
0.3625V
TO 0.75V
PV
FB
IN
3589 F03
SW
cancels the pole created by the
FB
L1
www.DataSheet4U.com
C
FB
R1
R2
C
OUT
3589p