LTM4601HVV View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTM4601HVV
Linear Technology
'LTM4601HVV' PDF : 30 Pages View PDF
LTM4601HV
APPLICATIONS INFORMATION
Layout Checklist/Example
The high integration of LTM4601HV makes the PCB board
layout very simple and easy. However, to optimize its electri-
cal and thermal performance, some layout considerations
are still necessary.
• Use large PCB copper areas for high current path, in-
cluding VIN, PGND and VOUT. It helps to minimize the
PCB conduction loss and thermal stress.
• Place high frequency ceramic input and output capaci-
tors next to the VIN, PGND and VOUT pins to minimize
high frequency noise.
• Place a dedicated power ground layer underneath the
unit. Refer frequency synchronization source to power
ground.
• To minimize the via conduction loss and reduce module
thermal stress, use multiple vias for interconnection
between top layer and other power layers.
• Do not put vias directly on pads unless they are capped.
• Use a separated SGND copper area for components
connected to signal pins. Connect the SGND to PGND
underneath the unit.
Figure 17 gives a good example of the recommended layout.
VIN
CIN CIN
GND
SIGNAL
GND
COUT
COUT
VOUT
4601HV F17
Figure 17. Recommended Layout (LGA and BGA
PCB Layouts Are Identical with the Exception of
Circle Pads for BGA. See Package Description.)
Frequency Adjustment
The LTM4601HV is designed to typically operate at 850kHz
across most input conditions. The fSET pin is normally
left open. The switching frequency has been optimized
for maintaining constant output ripple noise over most
operating ranges. The 850kHz switching frequency and
the 400ns minimum off time can limit operation at higher
duty cycles like 5V to 3.3V, and produce excessive induc-
tor ripple currents for lower duty cycle applications like
28V to 5V. The 5VOUT and 3.3VOUT drop out curves are
modified by adding an external resistor on the fSET pin to
allow for lower input voltage operation, or higher input
voltage operation.
Example for 5V Output
LTM4601HV minimum on-time = 100ns
tON = ((VOUT • 10pF)/IfSET), for VOUT > 4.8V use 4.8V
LTM4601HV minimum off-time = 400ns
tOFF = t – tON, where t = 1/Frequency
Duty Cycle = tON/t or VOUT/VIN
Equations for setting frequency:
IfSET = (VIN/(3 • RfSET)), for 28V operation, IfSET = 238µA,
tON = ((4.8 • 10pF)/IfSET), tON = 202ns, where the internal
RfSET is 39.2k. Frequency = (VOUT/(VIN • tON)) = (5V/(28
• 202ns)) ~ 884kHz. The inductor ripple current begins to
get high at the higher input voltages due to a larger voltage
across the inductor. This is noted in the Typical Inductor
Ripple Current vs Duty Cycle graph (Figure 3) where IL ≈
10A at 20% duty cycle. The inductor ripple current can be
lowered at the higher input voltages by adding an external
resistor from fSET to ground to increase the switching
frequency. A 7A ripple current is chosen, and the total
peak current is equal to 1/2 of the 7A ripple current plus
the output current. The 5V output current is limited to 8A,
so the total peak current is less than 11.5A. This is below
the 14A peak specified value. A 100k resistor is placed
from fSET to ground, and the parallel combination of 100k
and 39.2k equates to 28k. The IfSET calculation with 28k
and 28V input voltage equals 333µA. This equates to a
tON of 144ns. This will increase the switching frequency
from ~884kHz to ~1.24MHz for the 28V to 5V conversion.
20
4601hvfb
Share Link: 