LTC1625IGN#TR Linear Technology, LTC1625IGN#TR Datasheet - Page 9

• 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 basic LTC1625 application circuit is shown in Figure 1.
External component selection is primarily determined by
the maximum load current and begins with the selection of
the sense resistance and power MOSFETs. Because the
LTC1625 uses MOSFET V
is the R
and the inductor are chosen based largely on the desired
amount of ripple current. Finally, C
ability to handle the large RMS current into the converter
and C
output voltage ripple specification.
Power MOSFET Selection
The LTC1625 requires two external N-channel power
MOSFETs, one for the top (main) switch and one for the
bottom (synchronous) switch. Important parameters for
the power MOSFETs are the breakdown voltage V
threshold voltage V
transfer capacitance C
The gate drive voltage is set by the 5.2V INTV
Consequently, logic level threshold MOSFETs must be
used in LTC1625 applications. If low input voltage opera-
tion is expected (V
MOSFETs should be used. Pay close attention to the
V
the logic level MOSFETs are limited to 30V or less.
The MOSFET on-resistance is chosen based on the
required load current. The maximum average output cur-
rent I
the peak-to-peak ripple current I
current is inherently limited in a current mode controller
by the current threshold I
maximum V
mal conditions. The LTC1625 will not allow peak inductor
current to exceed 150mV/R
equation is a good guide for determining the required
R
lowing some margin for ripple current, current limit and
variations in the LTC1625 and external component values:
(BR)DSS
DS(ON)(MAX)
R
DS ON MAX
O(MAX)
OUT
(
DS(ON)
specification for the MOSFETs as well; many of
is chosen with low enough ESR to meet the
)(
is equal to the peak inductor current less half
DS
at 25 C (manufacturer’s specification), al-
of the MOSFETs. The operating frequency
sense voltage is about 150mV under nor-
)
IN
GS(TH)
U
< 5V), then sub-logic level threshold
I
O MAX
RSS
(
120
INFORMATION
DS
, on-resistance R
U
and maximum current I
TH
mV
sensing, the sense resistance
)
DS(ON)(TOP)
range. The corresponding
T
W
L
IN
. The peak inductor
is selected for its
. The following
DS(ON)
CC
U
, reverse
(BR)DSS
supply.
D(MAX)
,
.
The
variation in R
0.4%/ C as shown in Figure 2. Junction to case tempera-
ture T
maximum ambient temperature of 70 C, using
in the above equation is a reasonable choice. This equation
is plotted in Figure 3 to illustrate the dependence of
maximum output current on R
MOSFETs from Siliconix are shown as data points.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and
the load current. When the LTC1625 is operating in con-
tinuous mode, the duty cycles for the MOSFETs are:
Figure 3. Maximum Output Current vs R
T
JC
is a normalized term accounting for the significant
is around 10 C in most applications. For a
2.0
1.5
1.0
0.5
10
8
2
0
6
4
0
– 50
0
Figure 2. R
DS(ON)
Si4420
0.02
JUNCTION TEMPERATURE ( C)
0
Si4410
with temperature, typically about
DS(ON)
0.04
R
DS(ON)
Si4412
50
vs Temperature
0.06
( )
DS(ON)
100
0.08
DS(ON)
Si9936
. Some popular
1625 F03
1625 F02
LTC1625
0.10
150
at V
GS
80 C
= 4.5V
9
1.3