LTC1628IG-PG Linear Technology, LTC1628IG-PG Datasheet - Page 19

• Manufacturer Part Number: LTC1628IG-PG
• Description: IC REG SW 2PHASE STEPDOWN 28SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1628IG-PG

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: Adj to 0.8V
• Current - Output: 3A
• Frequency - Switching: 220kHz
• Voltage - Input: 3.5 ~ 30 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
Output Voltage
The LTC1628 output voltages are each set by an external
feedback resistive divider carefully placed across the
output capacitor. The resultant feedback signal is com-
pared with the internal precision 0.800V voltage reference
by the error amplifier. The output voltage is given by the
equation:
SENSE
The common mode input range of the current comparator
sense pins is from 0V to (1.1)INTV
operation is guaranteed throughout this range allowing
output voltage setting from 0.8V to 7.7V, depending upon
the voltage applied to EXTV
stage is biased with internal resistors from an internal
2.4V source as shown in the Functional Diagram. This
requires that current either be sourced or sunk from the
SENSE pins depending on the output voltage. If the output
voltage is below 2.4V current will flow out of both SENSE
pins to the main output. The output can be easily preloaded
by the V
comparator’s negative input bias current. The maximum
current flowing out of each pair of SENSE pins is:
Since V
can choose R1 in Figure 2 to have a maximum value to
absorb this current.
Regulating an output voltage of 1.8V, the maximum value
of R1 should be 32K. Note that for an output voltage above
2.4V, R1 has no maximum value necessary to absorb the
sense currents; however, R1 is still bounded by the
V
OSENSE
I
for V
R
V
SENSE
OUT
1
(
MAX
+
OSENSE
OUT
OUT
/SENSE
+
feedback current.
)
+ I
0 8
resistive divider to compensate for the current
< 2.4V
.
SENSE
24
is servoed to the 0.8V reference voltage, we
V
–
k
Pins
1
–
2 4
U
= (2.4V – V
.
R
R
2
V V
1
0 8
.
–
U
V
OUT
CC
. A differential NPN input
OUT
W
CC
)/24k
. Continuous linear
U
Soft-Start/Run Function
The RUN/SS1 and RUN/SS2 pins are multipurpose pins
that provide a soft-start function and a means to shut
down the LTC1628. Soft-start reduces the input power
source’s surge currents by gradually increasing the
controller’s current limit (proportional to V
can also be used for power supply sequencing.
An internal 1.2 A current source charges up the C
capacitor
reaches 1.5V, the particular controller is permitted to start
operating. As the voltage on RUN/SS increases from 1.5V
to 3.0V, the internal current limit is increased from 25mV/
R
up slowly, taking an additional 1.25s/ F to reach full
current. The output current thus ramps up slowly, reduc-
ing the starting surge current required from the input
power supply. If RUN/SS has been pulled all the way to
ground there is a delay before starting of approximately:
By pulling both RUN/SS pins below 1V and/or pulling the
STBYMD pin below 0.2V, the LTC1628 is put into low
current shutdown (I
driven directly from logic as shown in Figure 7. Diode D1
in Figure 7 reduces the start delay but allows C
up slowly providing the soft-start function. Each RUN/SS
pin has an internal 6V zener clamp (See Functional
Diagram).
SENSE
t
t
IRAMP
DELAY
*OPTIONAL TO DEFEAT OVERCURRENT LATCHOFF
3.3V OR 5V
to 75mV/R
.
When the voltage on RUN/SS1 (RUN/SS2)
LTC1628/LTC1628-PG
1 2
3
D1
1 5
Figure 7. RUN/SS Pin Interfacing
.
V
1 2
.
(a)
.
V
V
IN
A
1 5
SENSE
R
C
.
A
SS
Q
SS
*
V
RUN/SS
= 20 A). The RUN/SS pins can be
C
. The output current limit ramps
SS
C
1 25
SS
.
1 25
s
.
/
s
F C
/
INTV
SS
F C
CC
R
SS
(b)
ITH
*
SS
RUN/SS
SS
). This pin
to ramp
19
1628 F07
C
SS
1628fb
SS