Rainbow Electronics MAX15038 - Manual (Page 17)

MAX15038
after the voltage at FB increases above 92.5% of
V
REFIN . The additional delay prevents an early transi-
tion from monotonic startup to forced-PWM mode dur-
ing soft-start when a prolonged time constant external
REFIN voltage is applied.
The maximum allowed soft-start time is 2ms when an
external reference is applied at REFIN in the case of
starting up into prebiased output.
Skip Mode
Connect MODE to V DD to select skip mode. In skip
mode, the MAX15038 switches only as necessary to
maintain the output at light loads (not capable of sinking
current from the output), but still operates with fixed-fre-
quency (set by the resistor at FREQ terminal) PWM at
medium and heavy loads. This maximizes light-load effi-
ciency and reduces the input quiescent current.
In case of prolonged high-side idle activity (beyond
eight clock cycles), the low-side switch is turned on
briefly to rebuild the charge lost in the bootstrap capac-
itor before the next on-cycle of the high-side switch.
In skip mode, the low-side switch is turned off when the
inductor current decreases to 0.2A (typ) to ensure no
reverse current flowing from the output capacitor and
the best conversion efficiency/minimum supply current.
The high-side switch minimum on-time is controlled to
guarantee that 0.58A current is reached to avoid high
frequency bursts at no load conditions and that might
cause a rapid increase of the supply current caused by
additional switching losses.
Even if skip mode is selected at the device turn-on, the
monotonic startup mode is internally selected during
soft-start. The transition to skip mode is automatically
achieved 4096 clock cycles after the voltage at FB
increases above 92.5% of V
REFIN .
Changing from skip mode to forced-PWM mode and
vice-versa can be done at any time. The output capaci-
tor should be large enough to limit the output-voltage
overshoot/undershoot due to the settling times to reach
different duty-cycle set points corresponding to forced-
PWM mode and skip mode at light loads.
PCB Layout Considerations and
Thermal Performance
Careful PCB layout is critical to achieve clean and sta-
ble operation. It is highly recommended to duplicate the
MAX15038 EV kit layout for optimum performance. If devi-
ation is necessary, follow these guidelines for good PCB
layout:
1) Connect input and output capacitors to the power
ground plane; connect all other capacitors to the sig-
nal ground plane.
2) Place capacitors on V DD , V IN , and SS as close as
possible to the IC and its corresponding pin using
direct traces. Keep power ground plane (connected
to PGND) and signal ground plane (connected to
GND) separate.
3) Keep the high-current paths as short and wide as
possible. Keep the path of switching current short
and minimize the loop area formed by LX, the out-
put capacitors, and the input capacitors.
4) Connect IN, LX, and PGND separately to a large
copper area to help cool the IC to further improve
efficiency and long-term reliability.
5) Ensure all feedback connections are short and
direct. Place the feedback resistors and compensa-
tion components as close as possible to the IC.
6) Route high-speed switching nodes, such as LX,
away from sensitive analog areas (FB, COMP).
4A, 2MHz Step-Down Regulator
with Integrated Switches
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