MAX8576EUB Maxim Integrated Products, MAX8576EUB Datasheet - Page 13

• Manufacturer Part Number: MAX8576EUB
• Description: DC/DC Switching Controllers 3-28V Hystrtic Synch Step-Down Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX8576EUB

• Number Of Outputs: 1
• Output Voltage: 0.6 V to 25 V
• Output Current: 15 A
• Input Voltage: 3 V to 28 V
• Mounting Style: SMD/SMT
• Package / Case: uMAX-10
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

to SS. See Figure 2 for details. The maximum on-resis-
tance of the small external n-channel MOSFET should
be less than 40 so that the SS voltage is below 10mV.
Synchronous rectification reduces conduction losses in
the rectifier by replacing the normal Schottky catch
diode with a low-resistance MOSFET switch. The
MAX8576–MAX8579 also use the synchronous rectifier
to ensure proper startup of the boost gate-driver circuit.
The DL low-side waveform is always the complement of
the DH high-side drive waveform (with controlled dead
time to prevent cross-conduction or shoot-through). A
dead-time circuit monitors the DL output and prevents
the high-side MOSFET from turning on until DL is fully
off. For the dead-time circuit to work properly, there
must be a low-resistance, low-inductance path from the
DL driver to the MOSFET gate. Otherwise, the sense
circuitry in the MAX8576–MAX8579 may interpret the
MOSFET gate as off when gate charge actually
remains. Use very short, wide traces (50 mils to 100
mils wide if the MOSFET is 1in from the device). The
dead time at the other edge (DH turning off) is also
determined through gate sensing.
Gate-drive voltage for the high-side n-channel switch is
generated by a flying-capacitor boost circuit (Figure 4).
The capacitor between BST and LX is charged from the
IN supply up to V
side MOSFET is on. When the low-side MOSFET is
switched off, the stored voltage of the capacitor is
stacked above LX to provide the necessary turn-on
voltage (V
then closes an internal switch between BST and DH to
turn the high-side MOSFET on.
Figure 4. DH Boost Circuit
GS
High-Side Gate-Drive Supply (BST)
Synchronous-Rectifier Driver (DL)
) for the high-side MOSFET. The controller
MAX8576–
MAX8579
IN
______________________________________________________________________________________
minus the diode drop while the low-
IN
3V to 28V Input, Low-Cost, Hysteretic
BST
DH
DL
LX
Synchronous Step-Down Controllers
N
N
Current limit is set externally with a resistor from OCSET
to the drain of the high-side n-channel MOSFET that is
normally connected to the input supply. The resistor
programs the high-side peak current limit by setting the
maximum-allowed V
high-side MOSFET. An internal 50µA current sink sets
the maximum voltage drop relative to V
300mV, any overcurrent event (V
n-channel MOSFET is larger than the limit programmed
at OCSET) immediately sets DH low and terminates the
run cycle. If V
detected, DH is immediately set low and four sequential
overcurrent events terminate the run cycle. Once the
run cycle is terminated, the SS capacitor is slowly dis-
charged through the internal 250nA current sink to pro-
vide a hiccup current-limit effect. Choosing the proper
value resistor is discussed in the Setting the Current
Limit section.
Nominal switching frequency is programmable over the
200kHz to 500kHz range. This allows tradeoffs in effi-
ciency, switching frequency, inductor value, and com-
ponent size. Faster switching frequency allows for
smaller inductor values but does result in some efficien-
cy loss. Inductor-value calculations are provided in the
Inductor Value section. The switching frequency is
tuned by the selection of the feed-forward capacitor
(C
Thermal-overload protection limits total power dissipa-
tion in the MAX8576–MAX8579. When the junction tem-
perature exceeds T
sensor shuts down the IC, allowing the IC to cool. The
thermal sensor turns the IC on again after the junction
temperature cools to +140 C, resulting in a pulsed out-
put during continuous thermal-overload conditions.
Select an output voltage between 0.6V and 0.9 x V
connecting FB to a resistive voltage-divider between LX
and GND (see Figures 2 and 3). Choose R1 for approx-
imately 50µA to 150µA bias current in the resistive
divider. A wide range of resistor values is acceptable,
but a good starting point is to choose R1 as 6.04k .
Then, R3 is given by:
FF
). See the Feed-Forward Capacitor section.
FB
> 300mV and an overcurrent event is
Thermal-Overload Protection
Setting the Output Voltage
J
DS(ON)
= +160 C, an internal thermal
Design Procedures
Switching Frequency
Current-Limit Circuit
voltage drop across the
DS
of the high-side
IN
. If V
IN
FB
13
by
<