Detailed Description
The MAX1973/MAX1974 are 1.4MHz fixed-frequency
PWM current-mode step-down DC/DC converters. A
high 1.4MHz switching frequency allows use of small
inductors and small capacitors for filtering and decou-
pling. An internal synchronous rectifier improves effi-
ciency and eliminates the need for an external Schottky
freewheeling diode. On-chip current sensing uses the
on-resistance of the internal MOSFETs, eliminating cur-
rent-sensing resistors and improving efficiency.
The input voltage range is 2.6V to 5.5V. The output volt-
age is selectable to one of two presets, or adjustable by
using a resistor-divider. The output voltage of the
MAX1973 is preset to 1.8V or 2.5V by connecting FBSEL
to GND or IN, respectively. The MAX1974 is preset to
1.0V or 1.5V by connecting FBSEL to GND or IN, respec-
tively. In adjustable mode (see the Output Voltage
Selection section), the output voltage is programmable
down to 0.75V on the MAX1974, and down to 1.25V on
the MAX1973.
PWM Control Scheme
The MAX1973/MAX1974 use a fixed-frequency PWM
current-mode control scheme. The heart of the PWM
current-mode controller is an open-loop comparator
that compares the integrated voltage feedback signal
against the sum of the amplified current-sense signal
and the slope compensation ramp (see Figure 1). 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 a magnetic field.
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 (assuming that
the inductor value is relatively high to minimize ripple
current), the circuit acts as a switch-mode transconduc-
tance amplifier. It pushes the output LC filter pole, nor-
mally found in a voltage-mode PWM, to a higher
frequency. To preserve inner loop stability and eliminate
inductor staircasing, an internal slope-compensation
ramp is summed into the main PWM comparator.
During the second half of the switching cycle (off-time),
the internal high-side MOSFET turns off and the internal
low-side N-channel MOSFET turns on. The inductor
releases the stored energy as its current ramps down
while still 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. Under overload conditions, when the inductor
current exceeds the current limit, the high-side MOSFET
is not turned on at the rising edge of the clock, and the
low-side MOSFET remains on to let the inductor current
ramp down.
100% Duty-Cycle Operation
The MAX1973/MAX1974 can operate at 100% duty
cycle. In this state, the high-side P-channel MOSFET is
turned on (not switching). The dropout voltage in 100%
duty-cycle operation is the output current multiplied by
the sum of the on-resistance of the P-channel MOSFET
(R
DS(ON)P
) and the inductor resistance (R
L
).
V
DROPOUT
= I
OUT
✕ ( R
DS(ON)P
+ R
L
)
Current Sense and Current Limit
The current-sense circuit amplifies the current-sense
voltage generated by the high-side MOSFET’s on-resis-
tance and the inductor current (R
DS(ON)
✕ INDUCTOR).
This amplified current-sense signal and the internal
slope compensation signal are summed together at the
PWM comparator’s inverting input. The PWM compara-
tor turns off the internal high-side MOSFET when this
sum exceeds the integrated feedback voltage.
The internal high-side MOSFET has a current limit of
1.6A (typ). If the current flowing out of LX exceeds this
maximum, the high-side MOSFET turns off and the syn-
chronous rectifier MOSFET turns on. This lowers the
duty cycle and causes the output voltage to droop until
the current limit is no longer exceeded. There is also a
synchronous rectifier current limit of -0.85A, to protect
the device from current flowing into LX. If this negative
current limit is exceeded, the synchronous rectifier
turns off, and the inductor current continues to flow
through the high-side MOSFET body diode back to the
input until the beginning of the next cycle, or until the
inductor current drops to zero.
Soft-Start
To reduce the supply inrush current, soft-start circuitry
ramps up the output voltage during startup by charging
the SS capacitor with a 20µA current source. When SS
reaches its nominal value, the output is in full regula-
tion. The soft-start time (t
SS
) is determined from:
where V
SS
is the soft-start (reference) voltage (1.25V for
the MAX1973; 0.75V for the MAX1974), I
SS
is 20µA,
and C
SS
is the value of the capacitor connected to SS.
Soft-start occurs when power is first applied and when
the device exits shutdown. The part also goes through
