MAX1945REUI Maxim Integrated Products, MAX1945REUI Datasheet - Page 12

• Manufacturer Part Number: MAX1945REUI
• Description: DC/DC Switching Regulators 1MHz 1% 6A Step-Down PWM
• Manufacturer: Maxim Integrated Products

Specifications of MAX1945REUI

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The MAX1945R/MAX1945S employs digital soft-start to
reduce supply in-rush current during startup conditions.
When the device exits undervoltage lockout (UVLO),
shutdown mode, or restarts following a thermal-overload
event, the digital soft-start circuitry slowly ramps up the
voltages at REF and FB (see the Typical Operating
Characteristics). An internal oscillator sets the soft-start
time to 3.7ms (typ). Use a of 0.22µF capacitor (min) to
reduce the susceptibility to switching noise.
When V
switching. Once V
and the soft-start function activates.
A capacitor connected between BST and LX and a
Schottky diode connected from IN to BST generate the
gate drive for the internal high-side N-channel MOSFET.
When the low-side N-channel MOSFET is on, LX goes to
PGND. IN charges the bootstrap capacitor through the
Schottky diode. When the low-side N-channel MOSFET
turns off and the high side N-channel MOSFET turns on,
V
itor from discharging into IN.
The MAX1945R/MAX1945S operate in PWM mode with
a selectable fixed frequency or synchronized to an
external frequency. The devices switch at a frequency
of 500kHz when SYNC is connected to ground. The
devices switch at 1MHz with SYNC connected to V
Apply an external frequency of 400kHz to 1.2MHz with
10% to 90% duty cycle at SYNC to synchronize the
switching frequency of MAX1945R/MAX1945S.
The MAX1945R/MAX1945S feature selectable fixed and
adjustable output voltages. With FB connected to the
output, the output voltage is 1.8V when FBSEL is at
GND and 2.5V when FBSEL is at V
FBSEL is floating, connect FB to an external resistor
divider from V
voltage from 0.8V to 85% of V
the 1kΩ to 10kΩ range. Calculate R1 using the follow-
ing equation:
where V
1MHz, 1% Accurate, 6A Internal Switch
Step-Down Regulators
12
LX
goes to V
______________________________________________________________________________________
CC
FB
= 0.8V.
drops below 2.35V, the UVLO circuit inhibits
IN
. The Schottky diode prevents the capac-
Undervoltage Lockout (UVLO)
R
OUT
1
CC
=
R
Frequency Select (SYNC)
rises above 2.4V, UVLO clears
2
to GND to set the output



V
Output Voltage Select
V
OUT
FB
IN
(Figure 2). Select R2 in
−
Bootstrap (BST)
CC
1



(Figure 1). When
Soft-Start
CC
.
Drive CTL1 and CTL2 to ground to shut down the
MAX1945R/MAX1945S. In shutdown mode, the internal
MOSFETs stop switching and LX goes to high imped-
ance; REF and COMP go to ground.
The MAX1945R/MAX1945S provide selectable voltage
margining. The MAX1945R provides ±4% voltage mar-
gining, and the MAX1945S provides ±9% voltage margin-
ing. CTL1 and CTL2 set the voltage margins (Table 1).
Thermal-overload protection limits total power dissipa-
tion in the device. When the junction temperature (T
exceeds 165°C, a thermal sensor forces the device into
shutdown, allowing the die to cool. The thermal sensor
turns the device on again after the junction temperature
cools by 20°C, causing a pulsed output during continu-
ous overload conditions.The soft-start sequence begins
after a thermal-shutdown condition.
Because of the high switching frequency and tight out-
put tolerance, decouple V
V
the capacitor as close to VCC as possible.
Choose an inductor with the following equation:
where LIR is the ratio of the inductor ripple current to
average continuous current at a minimum duty cycle.
Choose LIR between 20% to 40% of the maximum load
current for best performance and stability.
Table 1. Setting Voltage Margin
Voltage Margin
CC
to GND with a 10Ω resistor from V
L
=
f
OSC
CTL1
V
V
0V
0V
CC
CC
V
×
OUT
V
IN
CTL2
V
V
0V
0V
CC
CC
×
CC
×
Design Procedure
(
LIR
V
with 0.1µF capacitor from
IN
Thermal Protection
MAX1945R
Voltage Margining
NOMINAL
−
×
+4%
V
OFF
-4%
Shutdown Mode
Inductor Design
V
OUT
I
OUT MAX
CC
)
CC
V
(
Decoupling
OUT
to IN. Place
MAX1945S
NOMINAL
)
+9%
OFF
-9%
J
)