Datasheet
MAX8597/MAX8598/MAX8599
Low-Dropout, Wide-Input-Voltage,
Step-Down Controllers
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Pin Description (continued)
PIN
MAX8597
MAX8598/
MAX8599
NAME
FUNCTION
17 15
FREQ
Frequency Adjust Input. Connect a resistor from FREQ to GND to set the switching frequency.
The range of the FREQ resistor is 14.3kΩ to 100kΩ (corresponding to 1400kHz to 200kHz).
18 —
AOUT
Output of the Uncommitted Operational Amplifier. AOUT is high impedance during
undervoltage lockout.
19 — AIN- Inverting Input of the Uncommitted Operational Amplifier
20 — AIN+ Noninverting Input of the Uncommitted Operational Amplifier
—16POK
Power-OK Output. POK is an open-drain output that goes high impedance when the regulator
output is greater than 88% of the regulation threshold. POK is low during shutdown.
— — EP Exposed Paddle. Connect to analog ground plane for improved thermal performance.
Detailed Description
The MAX8597/MAX8598/MAX8599 voltage-mode PWM
step-down controllers are designed to operate from
4.5V to 28V input and generate output voltages down to
0.6V. A proprietary switching algorithm stretches the
duty cycle to >99.5% for low-dropout design. Unlike
conventional step-down regulators using a p-channel
high-side MOSFET to achieve high duty cycle, the
MAX8597/MAX8598/MAX8599 drive n-channel
MOSFETs permitting high efficiency and high-current
designs.
The MAX8597 is available in a 20-pin thin QFN pack-
age and is designed for applications that use an ana-
log signal to control the output voltage with adjustable
offset, such as DC fan speed control. For example, a
12VDC fan can be driven from 6V to 12V with 12V input
power source depending on the system’s cooling
requirement to minimize fan noise and power consump-
tion. This is achieved with an internal uncommitted
operational amplifier. With the addition of an external
RC filter, a PWM input can also be used to control the
output voltage. The MAX8597 also generates a tracking
output for chipsets, ASICs, and DSP where core and
I/O supplies are split and require tracking. In applica-
tions where tighter output tolerance is required, the
MAX8597 output can be set by an external precision
reference source feeding to REFIN. The MAX8598/
MAX8599 are available in a 16-pin thin QFN package
and do not have the uncommitted operational amplifier,
reference input, and reference output, but offer a power-
OK output (POK). With the enable input and POK out-
put, the MAX8598/MAX8599 can easily be configured to
have power sequencing of multiple supply rails.
The MAX8597/MAX8598/MAX8599 allow startup with
prebias voltage on the output for applications where a
backup supply or a tracking device may charge the
output capacitor before the MAX8597/MAX8598/
MAX8599 are enabled. The MAX8599 has output over-
voltage protection.
These controllers feature lossless high-side peak
inductor current sensing, adjustable current limit, and
hiccup-mode short-circuit protection. Switching fre-
quency is set with an external resistor from 200kHz to
1.4MHz. This wide frequency range combined with a
wide-bandwidth error amplifier enable the loop-com-
pensation scheme to give the user ample flexibility to
optimize for cost, size, and efficiency.
DC-DC Controller
The MAX8597/MAX8598/MAX8599 step-down DC-DC
controllers use a PWM voltage-mode control scheme. An
internal high-bandwidth (25MHz) operational amplifier is
used as an error amplifier to regulate the output voltage.
The output voltage is sensed and compared with an inter-
nal 0.6V reference or REFIN (MAX8597) to generate an
error signal. The error signal is then compared with a
fixed-frequency ramp by a PWM comparator to give the
appropriate duty cycle to maintain output voltage regula-
tion. The high-side MOSFET turns on at the rising edge of
the internal clock 20ns after DL (the low-side MOSFET
gate drive) goes low. The high-side MOSFET turns off
once the internal ramp voltage reaches the error-amplifier
output voltage. The process repeats for every clock
cycle. During the high-side MOSFET on-time, current
flows from the input through the inductor to the output
capacitor and load. At the moment the high-side MOS-
FET turns off, the energy stored in the inductor during the
on-time is released to support the load as the inductor