LTC1622CMS8 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1622CMS8' PDF : 16 Pages View PDF
LTC1622
APPLICATIONS INFORMATION
down to 2.7V. Let’s assume that the MOSFET dissipation
is to be limited to PP = 250mW and its thermal resistance
is 50°C/W. Hence the junction temperature at TA = 25°C
will be 37.5°C and δp = 0.005 (37.5 – 25) = 0.0625. The
required RDS(ON) is then given by:
( ) ( ) RDS(ON) ≅
PP
2
= 0.11Ω
DC IOUT 1+ δp
The P-channel MOSFET requirement can be met by an
Si6433DQ.
The requirement for the Schottky diode is the most strin-
gent when VOUT = 0V, i.e., short circuit. With a 0.025Ω
RSENSE resistor, the short-circuit current through the
Schottky is 0.1/0.025 = 4A. An MBRS340T3 Schottky
diode is chosen. With 4A flowing through, the diode is
rated with a forward voltage of 0.4V. Therefore, the worst-
case power dissipated by the diode is 1.6W. The addition
of DFB (Figure 5) will reduce the diode dissipation to
approximately 0.8W.
The input capacitor requires an RMS current rating of at
least 0.75A at temperature, and COUT will require an ESR
of 0.1Ω for optimum efficiency.
PC Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1622. These items are illustrated graphically in the
layout diagram in Figure 6. Check the following in your
layout:
1. Is the Schottky diode closely connected between ground
at (–) lead of CIN and drain of the external MOSFET?
2. Does the (+) plate of CIN connect to the sense resistor
as closely as possible? This capacitor provides AC
current to the MOSFET.
3. Is the input decoupling capacitor (0.1µF) connected
closely between VIN (Pin 8) and ground (Pin 6)?
4. Connect the end of RSENSE as close to VIN (Pin 8) as
possible. The VIN pin is the SENSE + of the current
comparator.
5. Is the trace from the SENSE– (Pin 1) to the Sense
resistor kept short? Does the trace connect close to
RSENSE?
6. Keep the switching node, SW, away from sensitive
small signal nodes.
7. Does the VFB pin connect directly to the feedback
resistors? The resistive divider R1 and R2 must be
connected between the (+) plate of COUT and signal
ground. Optional capacitor C1 should be located as
close as possible to the LTC1622.
R1 and R2 should be located as close as possible to the
LTC1622. R2 should connect to the output as close to
the load as practicable.
+
VIN
RSENSE
CIN
RITH
CITH
R1
C1
R2
1 SENSE–
VIN 8
2 ITH
PDRV 7
LTC1622
3
6
VFB
GND
4 RUN/
SS
SYNC/ 5
MODE
CSS QUIET SGND
BOLD LINES INDICATE HIGH CURRENT PATHS
M1
0.1µF
SW
L1
VOUT
+
COUT
1622 F06
Figure 6. LTC1622 Layout Diagram (See PC Board Layout Checklist)
12
Share Link: 
