MAX15035 Datasheet, PDF (25/27 Pages) Maxim Integrated Products – 15A Step-Down Regulator with Internal Switches

15A Step-Down Regulator with Internal Switches
V_IN(MIN)
=
⎡
⎣⎢^1−
3.3V − 0V + 150mV ⎤
(1.5 × 350ns × 300kHz) _⎦⎥
=
3.74V
Calculating again with h = 1 gives the absolute limit of
dropout:
V_IN(MIN)
=
⎡
⎣⎢^1−
3.3V − 0V + 150mV ⎤
(1.0 × 350ns × 300kHz) _⎦⎥
=
3.52V
Therefore, V_IN must be greater than 3.52V, even with
very large output capacitance, and a practical input volt-
age with reasonable output capacitance would be 3.74V.
Applications Information
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:
1) Keep the high-current paths short, especially at the
ground terminals. This is essential for stable, jitter-
free operation.
2) Connect all analog grounds to a separate solid
copper plane, which connects to the AGND pin of
the Quick-PWM controller. This includes the V_CC
bypass capacitor, REF bypass capacitors, REFIN
components, and feedback compensation/dividers.
3) Keep the power traces and load connections short.
This is essential for high efficiency. The use of thick
copper PCB (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
milliohms of excess trace resistance causes a mea-
surable efficiency penalty.
4) Keep the power plane—especially LX—away from
sensitive analog areas (REF, REFIN, FB, ILIM).
Layout Procedure
1) Place the power components first, with ground ter-
minals adjacent (C_IN and C_OUT). If possible, make
all these connections on the top layer with wide,
copper-filled areas.
2) Make the DC-DC controller ground connections as
shown in Figure 1. This diagram can be viewed as
having four separate ground planes: input/output
ground, where all the high-power components go;
the power ground plane, where the PGND pin and
V_DD bypass capacitor go; the controller’s analog
ground plane where sensitive analog components,
the controller’s AGND pin, and V_CC bypass capaci-
tor go. The controller’s AGND plane must meet the
PGND plane only at a single point directly beneath
the IC. This point must also be very close to the out-
put capacitor ground terminal.
3) Connect the output power planes (V_CORE and sys-
tem ground planes) directly to the output filter
capacitor positive and negative terminals with multi-
ple vias. Place the entire DC-DC converter circuit
as close to the load as is practical.
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