Datasheet

LTC3676/LTC3676-1
15
3676fa
For more information www.linear.com/LTC3676
Figure 2. LDO1, LDO2 and LDO4 Application Circuit
OPERATION
LTC3676 Resistor Programmable LDO2 and LDO4
LDO2 and LDO4 output voltages are programmed by resis-
tor dividers
tied from the LDO output pin to the feedback
pin
as shown in Figure 2. The output voltage is calculated
using the following formula:
V
LDO
= 725 • 1+
R1
R2
⎛
⎝
⎜
⎞
⎠
⎟
mV
( )
output is 1.2V with selectable outputs of 2.5V, 2.8V, and
3.0V. LDO4 is enabled only through the command register
bit LDOB[2].
LDO4 Command Register Controls
Table 3. LDO4 Control Command Register Settings
COMMAND
REGISTER[BIT] VALUE SETTING
LDOB[0] 0*
1
Do Not Keep Alive LDO4 in Standby
Keep Alive LDO4 in Standby
LDOB[1]
0*
1
Enable LDO4 at Any Output V
oltage
Enable LDO4 Only if Output Voltage is <300mV
LDOB[2]
(LTC3676)
0*
1
LDO4 Disabled if EN_L4 is Low
LDO4 Enabled
LDOB[2]
(LTC3676-1)
0*
1
LDO4 Disabled
LDO4 Enabled
LDOB[4:3]
(LTC3676-1)
00*
LDO4 Output = 1.2V
LDOB[4:3]
(LTC3676-1)
01 LDO4 Output = 2.5V
LDOB[4:3]
(LTC3676-1)
10 LDO4 Output = 2.8V
LDOB[4:3]
(LTC3676-1)
11 LDO4 Output = 3V
*denotes default power-on value.
STEP-DOWN SWITCHING REGULATORS
The LTC3676 contains four buck regulators. Tw o of the
buck regulators are capable of delivering up to 2.5A load
current and the other two can deliver up to 1.5A each. The
regulators have forward and reverse current limiting, soft-
start, and switch slew rate control for lower radiated EMI.
The LTC3676 buck regulators are capable of 100% duty
cycle, or dropout, regulation. When in dropout the regulator
output voltage is equal to PV
IN
minus the load current times
R
DS(ON)
of the converters PMOS device and inductor DCR.
Each buck regulator is enabled using its enable pin or I
2
C
command register control. Operating modes, start-up op-
tion, reference
voltage, and switch slew rate are controlled
using the I
2
C port.
The buck converter I
2
C command register controls are
shown in Table 4, Table 5, Table 6, and Table 7.
+
–
V
IN
LDO
FB
R1
R2
3676 F02
1µF
0.725V
Fixed Output LDO3
Regulator LDO3 has a fixed voltage output of 1.8V.
Table 2. LDO2 and LDO3 Control Command Register Settings
COMMAND
REGISTER[BIT] VALUE SETTING
LDOA[0] 0*
1
Do Not Keep Alive LDO2 in Standby
Keep Alive LDO2 in Standby
LDOA[1]
0*
1
Enable LDO2 at Any Output V
oltage
Enable LDO2 Only if Output Voltage is <300mV
LDOA[2] 0*
1
LDO2 Disabled if EN_L2 is Low
LDO2 Enable
LDOA[3]
0*
1
Do Not Keep Alive LDO3 in Standby
Keep Alive LDO3 in Standby
LDOA[4]
0*
1
Enable LDO3 at Any Output V
oltage
Enable LDO3 0nly if Output Voltage is <300mV
LDOA[5] 0*
1
LDO3 Disabled if EN_L3 is Low
LDO3 Enabled
*denotes default power-on value.
LDO4 Operation LTC3676-1
LDO4 on the LTC3676-1 has neither enable nor feedback
pins. There are four LDO4 output voltages selectable by
command register bits LDOB[4:3]. The power-on default