MAX15023 Datasheet, PDF (13/28 Pages) Maxim Integrated Products – Wide 4.5V to 28V Input, Dual-Output Synchronous Buck Controller

Wide 4.5V to 28V Input, Dual-Output
Synchronous Buck Controller
The internal oscillator frequency is divided down to
obtain separated clock signals for each regulator. The
phase difference of the two clock signals is 180°, so that
the high-side MOSFETs turn on out-of-phase. The instan-
taneous input current peaks of both regulators no longer
overlap, resulting in reduced RMS ripple current and
input voltage ripple. As a result, this allows an input
capacitor with a lower ripple-current rating to be used or
allows the use of fewer or less expensive capacitors, as
well as reduces EMI filtering and shielding requirements.
Internal 5.2V Linear Regulator
The MAX15023’s internal functions and MOSFET drivers
are designed to operate from a 5V ±10% supply volt-
age. If the available supply voltage exceeds 5.5V, a
5.2V internal low-dropout linear regulator is used to
power internal functions and the MOSFET drivers at
V_CC. If an external 5V ±10% supply voltage is available,
then IN and V_CC can be tied to the 5V supply. The maxi-
mum regulator input voltage (V_IN) is 28V. The regulator’s
input (IN) must be bypassed to SGND with a 1μF
ceramic capacitor when the regulator is used. Bypass
the regulator’s output (V_CC) with a 4.7μF ceramic
capacitor to SGND. The V_CC dropout voltage is typically
70mV, so when V_IN is greater than 5.5V, V_CC is typically
5.2V. The MAX15023 also employs a UVLO circuit that
disables both regulators when V_CC falls below 3.8V
(typ). The 430mV UVLO hysteresis prevents chattering
on power-up/power-down.
The internal V_CC linear regulator can source up to
100mA to supply the IC, power the low-side gate dri-
vers, recharge the external boost capacitors, and sup-
ply small external loads. The current available for
external loads depends on the current consumed for
the MOSFET gate drive.
For example, when switched at 600kHz, a single
MOSFET with 18nC total gate charge (at V_GS = 5V)
requires 18nC x 600kHz ≅ 11mA. Since four MOSFETs
are driven and 6mA (max) is used by the internal con-
trol functions, the current available for external loads is:
(100 – (4 x 11) – 6)mA ≅ 50mA
MOSFET Gate Drivers (DH_, DL_)
The DH_ and DL_ drivers are optimized for driving
large size n-channel power MOSFETs. Under normal
operating conditions and after startup, the DL_ low-side
drive waveform is always the complement of the DH_
high-side drive waveform (with controlled dead time to
prevent cross-conduction or shoot-through). On each
channel, an adaptive dead-time circuit monitors the DH
and DL outputs and prevents the opposite-side
MOSFET from turning on until the other MOSFET is fully
off. Thus, the circuit allows the high-side driver to turn
on only when the DL_ gate driver has been turned off.
Similarly, it prevents the low-side (DL_) from turning on
until the DH_ gate driver has been turned off.
The adaptive driver dead time allows operation without
shoot-through with a wide range of MOSFETs, minimizing
delays, and maintaining efficiency. There must be a low-
resistance, low-inductance path from the DL_ and DH_
drivers to the MOSFET gates for the adaptive dead-time
circuits to work properly. Otherwise, because of the stray
impedance in the gate discharge path, the sense circuit-
ry could interpret the MOSFET gates as off while the V_GS
of the MOSFET is still high. To minimize stray imped-
ance, use very short, wide traces (50 mils to 100 mils
wide if the MOSFET is 1in from the driver).
Synchronous rectification reduces conduction losses in
the rectifier by replacing the normal low-side Schottky
catch diode with a low-resistance MOSFET switch. The
internal pulldown transistor that drives DL_ low is
robust, with a 0.75Ω (typ) on-resistance. This low on-
resistance helps prevent DL_ from being pulled up dur-
ing the fast rise time of the LX_ node, due to capacitive
coupling from the drain to the gate of the low-side syn-
chronous rectifier MOSFET.
High-Side Gate-Drive Supply (BST_)
and Internal Boost Switches
The high-side MOSFET is turned on by closing an inter-
nal switch between BST_ and DH_. This provides the
necessary gate-to-source voltage to turn on the high-side
MOSFET, an action that boosts the gate drive signal
above V_IN. The boost capacitor connected between
BST_ and LX_ holds up the voltage across the floating
gate driver during the high-side MOSFET on-time.
The charge lost by the boost capacitor for delivering the
gate charge is refreshed when the high-side MOSFET is
turned off and LX_ node swings down to ground. When
the corresponding LX_ node is low, an internal high-volt-
age switch connected between V_CC and BST_ recharges
the boost capacitor to the V_CC voltage. The need for
external boost diodes is negated. See the Boost Flying-
Capacitor Selection section in the Design Procedure
section to choose the right size of the boost capacitor.
Enable Inputs (EN_),
Adaptive Soft-Start and Soft-Stop
The MAX15023 can be used to regulate two indepen-
dent outputs. Each of the two outputs can be turned on
and off independently of one another by controlling the
enable input of each phase (EN1 and EN2).
A logic-high on each enable pin turns on the corre-
sponding channel. Then, the soft-start sequence is initi-
ated by step-wise increasing the reference voltage of
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