6A, 2MHz Step-Down Regulator
with Integrated Switches
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Figure 1. MAX8646 1MHz, All-Ceramic-Capacitor Design with V
OUT
= 1.8V
Detailed Description
The MAX8646 high-efficiency, voltage-mode switching
regulator is capable of delivering up to 6A of output
current. The MAX8646 provides output voltages from
0.6V to 0.9 x V
IN
from 2.35V to 3.6V input supplies,
making it ideal for on-board point-of-load applications.
The output voltage accuracy is better than ±1% over
load, line, and temperature.
The MAX8646 features a wide switching frequency
range, allowing the user to achieve all-ceramic-capacitor
designs and fast transient responses (see Figure 1). The
high operating frequency minimizes the size of external
components. The MAX8646 is available in a small (4mm
x 4mm), lead-free, 24-pin thin QFN package. The REFIN
function makes the MAX8646 an ideal candidate for
DDR and tracking power supplies. Using internal low-
R
DS(ON)
(23mΩ) n-channel MOSFETs for both high- and
low-side switches maintains high efficiency at both
heavy-load and high-switching frequencies.
The MAX8646 employs voltage-mode control architec-
ture with a high bandwidth (> 14MHz) error amplifier.
The voltage-mode control architecture allows up to
2MHz switching frequency, reducing board area. The
op-amp voltage-error amplifier works with type III com-
pensation to fully utilize the bandwidth of the high-fre-
quency switching to obtain fast transient response.
Adjustable soft-start time provides flexibilities to mini-
mize input startup inrush current. An open-drain,
power-good (PWRGD) output goes high when V
FB
reaches 90% of V
REFIN
or 0.54V.
Controller Function
The controller logic block is the central processor that
determines the duty cycle of the high-side MOSFET
under different line, load, and temperature conditions.
Under normal operation, where the current-limit and
temperature protection are not triggered, the controller
logic block takes the output from the PWM comparator
and generates the driver signals for both high-side and
low-side MOSFETs. The break-before-make logic and
the timing for charging the bootstrap capacitors are
calculated by the controller logic block. The error signal
from the voltage-error amplifier is compared with the
ramp signal generated by the oscillator at the PWM
comparator and, thus, the required PWM signal is pro-
duced. The high-side switch is turned on at the begin-
ning of the oscillator cycle and turns off when the ramp
Typical Application Circuit
