Rainbow Electronics MAX17008 User Manual
Page 33
In a single-phase configuration, the absolute point of
dropout is when the inductor current ramps down dur-
ing the minimum off-time (
ΔI
DOWN
) as much as it ramps
up during the on-time (
ΔI
UP
). The ratio h =
ΔI
UP
/
ΔI
DOWN
is an indicator of the ability to slew the inductor
current higher in response to increased load and must
always be greater than 1. As h approaches 1—the
absolute minimum dropout point—the inductor current
cannot increase as much during each switching cycle,
and V
SAG
greatly increases unless additional output
capacitance is used. A reasonable minimum value for h
is 1.5, but adjusting this up or down allows trade-offs
between V
SAG
, output capacitance, and minimum
operating voltage. For a given value of h, the minimum
operating voltage can be calculated as:
where V
CHG
is the parasitic voltage drop in the charge
path (see the
On-Time One-Shot
section), and t
OFF(MIN)
is from the
Electrical Characteristics
table. The absolute
minimum input voltage is calculated with h = 1.
If the calculated V
IN(MIN)
is greater than the required min-
imum input voltage, then reduce the operating frequency
or add output capacitance to obtain an acceptable V
SAG
.
If operation near dropout is anticipated, calculate V
SAG
to
be sure of adequate transient response.
Dropout Design Example:
V
OUT
= 1.5V
f
SW
= 300kHz
t
OFF(MIN)
= 250ns
V
CHG
= 150mV (10A load)
h = 1.5:
Calculating again with h = 1 gives the absolute limit of
dropout:
Therefore, V
IN
must be greater than 1.78V, even with
very large output capacitance, and a practical input volt-
age with reasonable output capacitance would be 2.0V.
PCB Layout Guidelines
Careful PCB layout is critical to achieve low switching
losses and clean, stable operation. The switching
power stage requires particular attention. If possible,
mount all the power components on the top side of the
board with their ground terminals flush against one
another. Follow these guidelines for good PCB layout:
•
Keep the high-current paths short, especially at the
ground terminals. This is essential for stable, jitter-
free operation.
•
Connect all analog grounds to a separate solid cop-
per plane, which connects to the GND pin of the
Quick-PWM controller. This includes the V
CC
bypass capacitor, REF bypass capacitors, REFIN1
components, and feedback compensation/dividers.
•
Keep the power traces and load connections short.
This is essential for high efficiency. The use of thick
copper PCBs (2oz vs. 1oz) can enhance full-load
efficiency by 1% or more. Correctly routing PCB
traces is a difficult task that must be approached in
terms of fractions of centimeters, where a single mil-
liohm of excess trace resistance causes a measur-
able efficiency penalty.
•
Keep the high current, gate-driver traces (DL, DH,
LX, and BST) short and wide to minimize trace
resistance and inductance. This is essential for
high-power MOSFETs that require low-impedance
gate drivers to avoid shoot-through currents.
•
When trade-offs in trace lengths must be made, it is
preferable to allow the inductor charging path to be
made longer than the discharge path. For example,
it is better to allow some extra distance between the
input capacitors and the high-side MOSFET than to
allow distance between the inductor and the low-
side MOSFET or between the inductor and the out-
put filter capacitor.
•
Route high-speed switching nodes away from sensi-
tive analog areas (REF, REFIN1, FB2, CSH, and CSL).
Layout Procedure
1) Place the power components first, with ground ter-
minals adjacent (low-side MOSFET source, C
IN
,
C
OUT
, and anode of the low-side Schottky). If possi-
ble, make all these connections on the top layer
with wide, copper-filled areas.
2) Mount the controller IC adjacent to the low-side
MOSFET. The DL gate traces must be short and
wide (50 mils to 100 mils wide if the MOSFET is 1in
from the controller IC).
V
V
mV
μs
kHz
IN MIN
(
)
.
( .
.
)
=
+
Ч
Ч
⎡
⎣
⎢
1 5
150
1 0 25
1 0
300
-
⎤⎤
⎦
⎥ = 1 78
.
V
V
V
mV
μs
kHz
IN MIN
(
)
.
( .
.
)
=
+
Ч
Ч
⎡
⎣
⎢
1 5
150
1 0 25
1 5
300
-
⎤⎤
⎦
⎥ = 1 86
.
V
V
V
V
h t
f
IN MIN
OUT
CHG
OFF MIN SW
(
)
(
)
=
×
(
)
⎛
⎝
⎜
⎜
⎞
⎠
⎟
+
-
1
⎟⎟
MAX17007A/MAX17008
Dual and Combinable QPWM Graphics
Core Controllers for Notebook Computers
______________________________________________________________________________________
33