LTM4600 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM4600' PDF : 24 Pages View PDF
LTM4600
applications information
Layout Checklist/Example
The high integration of the LTM4600 makes the PCB
board layout very simple and easy. However, to optimize
its electrical and thermal performance, some layout con-
siderations 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
• 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 a via directly on pad unless it is capped
• Use a separated SGND ground copper area for com-
ponents connected to signal pins. Connect the SGND
to PGND underneath the unit
Figure 16 gives a good example of the recommended layout.
LTM4600 Frequency Adjustment
The LTM4600 is designed to typically operate at 850kHz
across most input and output conditions. The control ar-
chitecture is constant on time valley mode current control.
The fADJ pin is typically left open or decoupled with an
optional 1000pF capacitor. The switching frequency has
been optimized to maintain constant output ripple over the
operating conditions. The equations for setting the operat-
ing frequency are set around a programmable constant on
time. This on time is developed by a programmable current
into an on board 10pF capacitor that establishes a ramp
that is compared to a voltage threshold equal to the output
voltage up to a 2.4V clamp. This ION current is equal to: ION
= (VIN – 0.7V)/110k, with the 110k onboard resistor from
VIN to fADJ. The on time is equal to tON = (VOUT/ION) • 10pF
and tOFF = ts – tON. The frequency is equal to: Freq. = DC/
tON. The ION current is proportional to VIN, and the regula-
tor duty cycle is inversely proportional to VIN, therefore
the step-down regulator will remain relatively constant
frequency as the duty cycle adjustment takes place with
lowering VIN. The on time is proportional to VOUT up to a
2.4V clamp. This will hold frequency relatively constant
with different output voltages up to 2.4V. The regulator
switching period is comprised of the on time and off time
as depicted in Figure 17.
VIN
SGND
CIN
PGND
LOAD
VOUT
TOP LAYER
4600 F16
Figure 16. Recommended PCB Layout
For more information www.linear.com/LTM4600
4600fd
17
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