LTC1625IGN#TR Linear Technology, LTC1625IGN#TR Datasheet - Page 10

• Manufacturer Part Number: LTC1625IGN#TR
• Description: IC SW REG STEP-DOWN SYNC 16-SSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LTC1625IGN#TR

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 1
• Voltage - Output: 1.19 ~ 36 V
• Current - Output: 50mA
• Frequency - Switching: 150kHz
• Voltage - Input: 3.7 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Power - Output: -
• Other names: LTC1625IGNTR; Q1127755

APPLICATIONS
The MOSFET power dissipations at maximum output
current are:
Both MOSFETs have I
includes an additional term for transition losses, which are
largest at high input voltages. The constant k = 1.7 can be
used to estimate the amount of transition loss. The bottom
MOSFET losses are greatest at high input voltage or during
a short circuit when the duty cycle is nearly 100%.
Operating Frequency and Synchronization
The choice of operating frequency and inductor value is a
trade-off between efficiency and component size. Low
frequency operation improves efficiency by reducing
MOSFET switching losses, both gate charge loss and
transition loss. However, lower frequency operation
requires more inductance for a given amount of ripple
current.
The internal oscillator runs at a nominal 150kHz frequency
when the SYNC pin is left open or connected to ground.
Pulling the SYNC pin above 1.2V will increase the fre-
quency by 50%. The oscillator will injection lock to a clock
signal applied to the SYNC pin with a frequency between
165kHz and 200kHz. The clock high level must exceed
1.2V for at least 1 s and no longer than 4 s as shown in
Figure 4. The top MOSFET turn-on will synchronize with
the rising edge of the clock.
LTC1625
10
Bottom Duty Cycle
P
P
Top Duty Cycle
BOT
TOP
V
V
( )(
V
OUT
k V
IN
IN
V
–
IN
IN
V
U
(
OUT
I
2
O MAX
)(
V
V
(
OUT
2
I
IN
O MAX
R losses and the P
INFORMATION
V
(
(
I
U
IN
O MAX
)
2
(
V
–
)(
IN
)
)(
V
T TOP
OUT
C
)
2
(
RSS
)(
W
)
)( )
T BOT
)(
(
f
R
DS ON
)
TOP
)(
(
R
U
DS ON
)
equation
)
(
)
)
Inductor Value Selection
Given the desired input and output voltages, the inductor
value and operating frequency directly determine the
ripple current:
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with small ripple current. To achieve this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
current occurs at the highest V
current does not exceed a specified maximum, the induc-
tor should be chosen according to:
Burst Mode Operation Considerations
The choice of R
the load current at which the LTC1625 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
L
I
BURST PEAK
I
L
( )(
f
0
(
( )( )
V
f L
OUT
V
I
OUT
L MAX
Figure 4. SYNC Clock Waveform
(
DS(ON)
)
1 s
1
R
7V
1.2V
)
30
–
)
DS ON
O(MAX)
and inductor value also determines
mV
V
(
V
OUT
1
IN
–
)
. Note that the largest ripple
V
IN
IN MAX
V
OUT
. To guarantee that ripple
(
4 s
)
1625 F04