16
LTC1530
1530fa
Table 3. Suggested Compensation Network for a 5V Input
Application Using Multiple Paralleled 1500µF SANYO MV-GX
Output Capacitors for 2.5V Output
L
O
(µH) C
O
(µF) R
C
(kΩ)C
C
(µF) C1 (pF)
1 4500 3 0.022 470
1 6000 4 0.022 330
1 9000 6 0.022 220
2.7 4500 8.2 0.022 150
2.7 6000 11 0.01 100
2.7 9000 16 0.01 100
5.6 4500 16 0.01 100
5.6 6000 22 0.01 68
5.6 9000 33 0.01 47
Note: For different values of V
OUT
, multiply the R
C
value by V
OUT
/2.5 and
multiply the C
C
and C1 values by 2.5/V
OUT
. This maintains the same
crossover frequency for the closed-loop transfer function.
Thermal Considerations
Limit the LTC1530’s junction temperature to less than
125°C. The LTC1530’s SO-8 package is rated at 130°C/W
and care must be taken to ensure that the worst-case input
voltage and gate drive load current requirements do not
cause excessive die temperatures. Short-circuit or fault
conditions may activate the internal thermal shutdown
circuit.
LAYOUT CONSIDERATIONS
When laying out the printed circuit board (PCB), the
following checklist should be used to ensure proper
operation of the LTC1530. These items are illustrated
graphically in the layout diagram of Figure 9. The thicker
lines show the high current power paths. Note that at 10A
current levels or above, current density in the PCB itself is
a serious concern. Traces carrying high current should be
as wide as possible. For example, a PCB fabricated with
2oz copper requires a minimum trace width of 0.15" to
carry 10A, and only if trace length is kept short.
1. In general, begin the layout with the location of the
power devices. Orient the power circuitry so that a
clean power flow path is achieved. Maximize conduc-
tor widths but minimize conductor lengths. Keep high
current connections on one side of the PCB if possible.
If not, minimize the use of vias and keep the current
density in the vias to <1A/via, preferably <0.5A/via.
After achieving a satisfactory power path layout, pro-
ceed with the control circuitry layout. It is much easier
to find routes for the relatively small traces in the
control circuits than it is to find circuitous routes for
high current paths.
2. Tie the GND pin to the ground plane at a single point,
preferably at a fairly quiet point in the circuit, such as the
bottom of the output capacitors. However, this is not
always practical due to physical constraints. Connect
the low side source to the input capacitor ground.
Connect the input and output capacitor to the ground
plane. Run a separate trace for the low side FET source
to the input capacitors. Do not tie this single point
ground in the trace run between the low side FET source
and the input capacitor ground. This area of the ground
plane is very noisy.
3. Locate the small signal resistor and capacitors used for
frequency compensation close to the COMP pin. Use a
separate ground trace for these components that ties
directly to the GND pin of the LTC1530. Do not connect
these components to the ground plane!
4. Place the PV
CC
decoupling capacitor as close to the
LTC1530 as possible. The 10µF bypass capacitor shown
at PV
CC
helps provide optimum regulation performance
by minimizing ripple at the PV
CC
pin.
5. Connect the (+) plate of C
IN
as close as possible to the
drain of the upper MOSFET. LTC recommends an
additional 1µF low ESR ceramic capacitor between V
IN
and power ground.
6. The V
SENSE
/V
OUT
pin is very sensitive to pickup from the
switching node. Care must be taken to isolate this pin
from capacitive coupling to the high current inductor
switching signals. A 0.1µF is recommended between
the V
OUT
pin and the GND pin directly at the LTC1530 for
fixed voltage versions. For the adjustable voltage ver-
sion, keep the resistor divider close to the LTC1530.
The bottom resistor’s ground connection should tie
directly to the LTC1530’s GND pin.
7. Kelvin sense I
MAX
and I
FB
at the drain and source pins
of Q1.
8. Minimize the length of the gate lead connections.
APPLICATIO S I FOR ATIO
WUUU
