LTC1143LCS-ADJ#PBF Linear Technology, LTC1143LCS-ADJ#PBF Datasheet - Page 8

• Manufacturer Part Number: LTC1143LCS-ADJ#PBF
• Description: IC SW REG STEP-DOWN DUAL 16-SOIC
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1143LCS-ADJ#PBF

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 2
• Voltage - Output: Adjustable
• Current - Output: 50mA
• Frequency - Switching: 400kHz
• Voltage - Input: 3.5 ~ 16 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

OPERATION
APPLICATIONS
The basic LTC1143L-ADJ application circuit is shown in
Figure 1. The LTC1143 and LTC1143L are similar but omit
the external resistive V
13). External component selection is driven by the load
requirement and begins with V
R
Next, the power MOSFET and D1 are selected. Finally, C
and C
the two regulator sections are identical, the process of
component selection is the same for both sections. The
circuit shown in Figure 1 can be configured for operation
up to an input voltage of 16V.
Output Voltage Selection
The LTC1143/LTC1143L output voltages are internally set
to 3.3V and 5V. The LTC1143L-ADJ requires an external
resistive divider from V
shown in Figure 1. The regulated LTC1143L-ADJ output
voltages are given by:
To prevent stray pickup, a 100pF capacitor is suggested
across R1 and R3 located close to the LTC1143L-ADJ.
LTC1143/LTC1143L
LTC1143L-ADJ
The circuit now enters sleep mode with the power MOSFET
turned off. In sleep mode a majority of the circuitry is
turned off, dropping the quiescent current from 1.6mA to
160 A (for one regulator block). The load current is now
being supplied from the output capacitor. When the output
voltage has dropped by the amount of hysteresis in
comparator V, the P-channel MOSFET is again turned on
and the process repeats.
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset (V
8
SENSE
V
V
OUT
OUT
OUT
. Once R
1
2
are selected and the loop is compensated. Since
1 25 1
1 25 1
SENSE
U
U
R
R
R
R
is known, C
Refer to Functional Diagram and Figure 1
2
1
4
3
OUT
OUT
INFORMATION
U
dividers (see Figures 10 and
to V
OUT
T
FB
W
and L can be chosen.
and the selection of
on each section as
OS
) is incorpo-
U
IN
For Figure 1 applications with V
R
inputs operate near ground. When the current comparator
is operated at less than 2V common mode, the off-time
increases approximately 40%, requiring the use of a
smaller timing capacitor C
R
R
The LTC1143 series current comparators have a threshold
range that extends from a minimum of 25mV/R
maximum of 150mV/R
threshold sets the peak of the inductor ripple current,
yielding a maximum output current I
value less half the peak-to-peak ripple current. For proper
Burst Mode operation, I
equal to the minimum current comparator threshold.
Since efficiency generally increases with ripple current,
the maximum allowable ripple current is assumed, i.e.,
I
Operating Frequency). Solving for R
margin for variations in the LTC1143 series and external
component values yields:
A graph for selecting R
current is given in Figure 2.
rated in the gain stage. This prevents the current compara-
tor threshold from increasing until the output voltage has
dropped below a minimum threshold.
Using constant off-time architecture the operating fre-
quency is a function of the input voltage. To minimize the
frequency variation as dropout is approached, the off-time
controller increases the C
below V
turned on continuously (100% duty cycle), providing
extremely low dropout operation.
RIPPLE(P-P)
SENSE
SENSE
SENSE
R
SENSE
is moved to ground, the current sense comparator
Selection for Output Current
is chosen based on the required output current.
OUT
= 100mV
= 25mV/R
+ 1.5V. In dropout the P-channel MOSFET is
I
MAX
SENSE
SENSE
RIPPLE(P-P)
SENSE
T
T
discharge current as V
.
. (See C
. The current comparator
versus maximum output
OUT
must be less than or
MAX
T
SENSE
below 2V, or when
and L Selection for
equal to the peak
and allowing a
SENSE
IN
drops
to a