MAX1945SEUI Maxim Integrated Products, MAX1945SEUI Datasheet - Page 11

• Manufacturer Part Number: MAX1945SEUI
• Description: DC/DC Switching Regulators 1MHz 1% 6A Step-Down PWM
• Manufacturer: Maxim Integrated Products

Specifications of MAX1945SEUI

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

The MAX1945R/MAX1945S high-efficiency PWM
switching regulators deliver up to 6A of output current.
The devices operate at a selectable fixed frequency
(500kHz or 1MHz) or can be synchronized to an exter-
nal frequency (400kHz to 1.2MHz). The devices oper-
ate from a 2.6V to 5.5V input supply voltage and have a
selectable output voltage of 1.8V or 2.5V, or an
adjustable output voltage from 0.8V to 85% of the input
voltage, making the MAX1945R/MAX1945S ideal for on-
board post-regulation applications. The high switching
frequency allows the use of small external components.
Internal synchronous rectifiers improve efficiency and
eliminate the typical Schottky freewheeling diode. Total
output error over load, line, and temperature is less
than ±1%.
The MAX1945R/MAX1945S step-down converters use a
PWM current-mode control scheme. A PWM comparator
compares the integrated voltage-feedback signal
against the sum of the amplified current-sense signal
and the slope-compensation ramp. At each rising edge
of the internal clock, the internal high-side MOSFET
turns on until the PWM comparator trips. During this on-
time, current ramps up through the inductor, sourcing
current to the output and storing energy in the inductor.
The current-mode feedback system regulates the peak
inductor current as a function of the output voltage
error signal. Because the average inductor current is
nearly the same as the peak inductor current (<30%
ripple current), the circuit acts as a switch-mode
transconductance amplifier.
To preserve inner-loop stability and eliminate inductor
staircasing, a slope-compensation ramp is summed into
the main PWM comparator. During the off-cycle, the
internal high-side N-channel MOSFET turns off, and the
internal low-side N-channel turns on. The inductor releas-
es the stored energy as its current ramps down while still
19, 21, 23,
PIN
25
27
28
NAME
PGND
CTL1
CTL2
EP
______________________________________________________________________________________
1MHz, 1% Accurate, 6A Internal Switch
Detailed Description
Power Ground. Connect all PGND together close to the device. Star connect GND to PGND (see the PC
Board Layout Considerations section).
Output Margining Control Inputs. When CTL1 = CTL2 = GND, the regulator is off. When CTL1 = CTL2 =
V
to the margin-low output (-4% or -9%). When CTL1 = GND and CTL2 = V
margin-high output (+4% or +9%).
Exposed Pad. Connect to PGND to improve power dissipation.
CC
, the regulator runs at nominal output voltage. When CTL1 = V
Controller Function
providing current to the output. The output capacitor
stores charge when the inductor current exceeds the
load current and discharges when the inductor current is
lower, smoothing the voltage across the load. During an
overload condition, when the inductor current exceeds
the current limit (see the Current Limit section), the high-
side MOSFET does not turn on at the rising edge of the
clock, and the low-side MOSFET remains on to let the
inductor current ramp down.
An internal current-sense amplifier produces a current
signal proportional to the voltage generated by the high-
side MOSFET on-resistance and the inductor current
(R
the internal slope-compensation signal sum together at
the comparator inverting input. The PWM comparator
turns off the internal high-side MOSFET when this sum
exceeds the COMP voltage from the error amplifier.
The internal high-side MOSFET has a current limit of
8A (min). If the current flowing out of LX exceeds this limit,
the high-side MOSFET turns off and the synchronous rec-
tifier turns on. This lowers the duty cycle and causes the
output voltage to droop until the current limit is no longer
exceeded. The minimum duty cycle is limited to 10%. A
synchronous rectifier current limit of 2A minimum protects
the device from current flowing into LX.
When the negative current limit is exceeded, the device
turns off the synchronous rectifier, forcing the inductor
current to flow through the high-side MOSFET body
diode and back to the input, until the beginning of the
next cycle, or until the inductor current drops to zero.
The MAX1945R/MAX1945S use a pulse-skip mode to
prevent overheating during short-circuit output condi-
tions. The device enters pulse-skip mode when the FB
voltage drops below 300mV, limiting the current and
reducing power dissipation. Normal operation resumes
upon removal of the short-circuit condition.
Step-Down Regulators
DS(ON)
FUNCTION
✕
Pin Description (continued)
I
LX
). The amplified current-sense signal and
CC
and CTL2 = GND, the output is set
CC
, the output is set to the
Current Sense
Current Limit
11