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General Description

The MAX8597/MAX8598/MAX8599 voltage-mode PWM

step-down controllers are designed to operate from a

4.5V to 28V input supply and generate output voltages

down to 0.6V. A proprietary switching algorithm stretch-

es 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 resulting in high efficiency and high-current-

capability designs.

The MAX8597 is available in a 20-pin thin QFN pack-

age and is designed for applications that use an analog

signal to control the output voltage with an adjustable

offset, such as DC fan-speed control. This is achieved

with an internal uncommitted operational amplifier. The

MAX8597 is also targeted for tracking output-voltage

applications for chipsets, ASIC and DSP cores, and I/O

supplies. The MAX8598/MAX8599 are available in a 16-

pin thin QFN package and do not have the uncommitted

operational amplifier, reference input, and reference out-

put, but offer an open-drain, power-OK output.

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. In addition, the MAX8599 fea-

tures output overvoltage protection.

These controllers also 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 enables the loop com-

pensation scheme to give the user ample flexibility to

optimize for cost, size, and efficiency.

Applications

Nonisolated Power Modules

Variable-Speed DC Fan Power Supplies

(MAX8597)

Tracking Power Supplies (MAX8597)

Chipset Power Supplies

Features

♦ Low Dropout with >99.5% Duty Cycle

♦ Lossless High-Side Current Limit

♦ Wide 4.5V to 28V Input Range

♦ Dynamic Output Voltage Adjustment with

Adjustable Offset (MAX8597)

♦ Remote Voltage Sensing for Both Positive and

Negative Rails (MAX8597)

♦ Tracking Output Through REFIN (MAX8597)

♦ Adjustable Switching Frequency from 200kHz to

1.4MHz

♦ Adjustable Soft-Start

♦ Prebias Startup

♦ Enable and Power-OK (MAX8598/MAX8599) for

Flexible Sequencing

♦ 25MHz Error Amplifier

♦ Adjustable Hiccup Current Limit for Output

Short-Circuit Protection

♦ Output Overvoltage Protection (MAX8599)

♦ Small, Low-Profile Thin QFN Package

MAX8597/MAX8598/MAX8599

Low-Dropout, Wide-Input-Voltage,

Step-Down Controllers

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

19-3505; Rev 1; 6/05

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

EVALUATION KIT

AVAILABLE

PART TEMP RANGE PIN-PACKAGE

MAX8597ETP -40°C to +85°C

20 Thin QFN 4mm x 4mm

(T2044-3)

MAX8597ETP+ -40°C to +85°C

20 Thin QFN 4mm x 4mm

(T2044-3)

MAX8598ETE -40°C to +85°C

16 Thin QFN 4mm x 4mm

(T1644-4)

MAX8598ETE+ -40°C to +85°C

16 Thin QFN 4mm x 4mm

(T1644-4)

MAX8599ETE -40°C to +85°C

16 Thin QFN 4mm x 4mm

(T1644-4)

MAX8599ETE+ -40°C to +85°C

16 Thin QFN 4mm x 4mm

(T1644-4)

+Denotes lead-free package.

Pin Configurations appear at end of data sheet.

Summary of content (24 pages)

PAGE 1
9-3505; Rev 1; 6/05 KIT ATION EVALU E L B A AVAIL Low-Dropout, Wide-Input-Voltage, Step-Down Controllers The MAX8597/MAX8598/MAX8599 voltage-mode PWM step-down controllers are designed to operate from a 4.5V to 28V input supply and generate output voltages down to 0.6V. A proprietary switching algorithm stretches the duty cycle to >99.5% for low-dropout design.
PAGE 2
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers ABSOLUTE MAXIMUM RATINGS V+, ILIM to GND .....................................................-0.3V to +30V AVL, VL to GND........................................................-0.3V to +6V PGND to GND .......................................................-0.3V to +0.3V FB, EN, POK, AIN-, AIN+, REFIN to GND ................-0.3V to +6V AOUT, REFOUT, FREQ, SS, COMP to GND .....................................................-0.
PAGE 3
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers (VV+ = VVL = VAVL = VEN = VREFIN = 5V, VBST = 6V, VLX = 1V, CVL = 4.7µF, CREFOUT = 1µF, VAIN- = VAOUT, VAIN+ = 2.5V, VILIM = VLX - 0.2V, VFB = 0.65V, GND = PGND = 0V, CSS = 0.01µF, RFREQ = 20kΩ, TA = 0°C to +85°C, typical values are at TA = +25°C, unless otherwise noted.) PARAMETER CONDITIONS DH Minimum Off-Time MIN TYP MAX UNITS 180 200 220 ns 115 140 ns 100 nA 0.600 0.
PAGE 4
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers ELECTRICAL CHARACTERISTICS (continued) (VV+ = VVL = VAVL = VEN = VREFIN = 5V, VBST = 6V, VLX = 1V, CVL = 4.7µF, CREFOUT = 1µF, VAIN- = VAOUT, VAIN+ = 2.5V, VILIM = VLX - 0.2V, VFB = 0.65V, GND = PGND = 0V, CSS = 0.01µF, RFREQ = 20kΩ, TA = 0°C to +85°C, typical values are at TA = +25°C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS 0 2.75 V VAVL 1.0 VAVL 0.
PAGE 5
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers (VV+ = VVL = VAVL = VEN = VREFIN = 5V, VBST = 6V, VLX = 1V, CVL = 4.7µF, CREFOUT = 1µF, VAIN- = VAOUT, VAIN+ = 2.5V, VILIM = VLX - 0.2V, VFB = 0.65V, GND = PGND = 0V, CSS = 0.01µF, RFREQ = 20kΩ, TA = -40°C to +85°C, typical values are at TA = +25°C, unless otherwise noted.) (Note 1) PARAMETER CONDITIONS MIN TYP MAX UNITS FREQUENCY Frequency RFREQ = 100kΩ 140 240 RFREQ = 20.0kΩ 800 1200 RFREQ = 14.
PAGE 6
ELECTRICAL CHARACTERISTICS (continued) (VV+ = VVL = VAVL = VEN = VREFIN = 5V, VBST = 6V, VLX = 1V, CVL = 4.7µF, CREFOUT = 1µF, VAIN- = VAOUT, VAIN+ = 2.5V, VILIM = VLX - 0.2V, VFB = 0.65V, GND = PGND = 0V, CSS = 0.01µF, RFREQ = 20kΩ, TA = -40°C to +85°C, typical values are at TA = +25°C, unless otherwise noted.
PAGE 7
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers STARTUP/SHUTDOWN WITH EN (ILOAD = 20A) OUTPUT PREBIASED STARTUP MAX8597 toc07 MAX8597 toc08 5V/div VPOK 10V/div VLX 1.2V ILX 10A/div 1.0V VOUT 5V/div 1V/div VOUT VIN 5V/div VDL 5V/div VEN 1ms/div 2ms/div ENTERING DROPOUT WAVEFORMS CIRCUIT OF FIGURE 1 OUTPUT VOLTAGE vs.
PAGE 8
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers Typical Operating Characteristics (continued) (Circuit of Figure 4, TA = +25°C, 500kHz switching frequency, VIN = 12V, unless otherwise noted.
PAGE 9
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers ILIM MAX8597 MAX8598 MAX8599 200µA BST DH FREQ CONTROL LOGIC OSC LX 1VP-P PWM VL 1/20 COUNTER DL PGND AVL REFERENCE REFOUT (MAX8597) SOFT-START EN BIAS VREG COMP V+ EAMP VL VL GND 1.17 x VREG UVP SS REFIN (MAX8597) OVP (MAX8599) 0.7 x VREG FB POK (MAX8598/ MAX8599) AOUT (MAX8597) N AIN+ (MAX8597) 0.
PAGE 10
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers Pin Description PIN MAX8597 MAX8598/ MAX8599 NAME 1 1 AVL 2 — REFIN 3 2 GND 4 3 SS Soft-Start Programming Input. Connect a capacitor from SS to GND to set the soft-start time. See the Selecting the Soft-Start Capacitor section for details. 5 4 FB Feedback Input. Connect to the center tap of an external resistor-divider to set the output voltage. Regulates to 0.
PAGE 11
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers 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.
PAGE 12
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers current ramps down through the low-side MOSFET body diode; 20ns after DH goes low, the low-side MOSFET turns on, resulting in a lower voltage drop to increase efficiency. The low-side MOSFET turns off at the rising edge of the next clock pulse, and when its gate voltage discharges to zero, the high-side MOSFET turns on and another cycle starts.
PAGE 13
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers The power-OK signal (POK) is an open-drain output that goes high impedance when FB is above 91% of its nominal threshold. There is an eight clock-cycle delay before POK goes high impedance. For 500kHz switching frequency, this delay is typically 16µs. To obtain a logic voltage output, connect a pullup resistor from POK to AVL. A 100kΩ resistor works well for most applications. If unused, connect POK to GND or leave it unconnected.
PAGE 14
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers Design Procedure VIN (10.8V TO 13.2V) R15 3Ω R1 33.2kΩ C2A 10µF C1 1µF VL EN ON OFF D1 CMDSH-3 AVL R4 100kΩ VADJ (0V TO 5V) C15 0.01µF 19 R5 32.4kΩ 20 C6 0.033µF 4 R6 48.7kΩ 18 R9 6.04kΩ R7 48.7kΩ R10 93.1kΩ R8 24.9kΩ 5 C8 4.7pF R12 47kΩ R11 5.1kΩ 2 7 REFIN EN 17 9 FREQ V+ 16 ILIM AIN- BST SS C3 0.01µF 13 AIN+ DH C2B 10µF R2 1.21kΩ Q1 IRF7821 14 C5 0.
PAGE 15
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers VIN (10.8V TO 13.2V) R1 20kΩ ON OFF R12 3Ω EN C2A 10µF C1 1µF VL REFIN R3 70kΩ C4 1000pF D2 CMPD914 19 R4 18.2kΩ R5 6.98kΩ 2 7 17 REFIN EN FREQ 9 V+ 16 ILIM AIN- 20 D1 CMDSH-3 BST AIN+ DH Q1 IRF7807Z 14 C5 0.22µF 4 DL FB PGND AVL 1 COMP GND 3 6 C8 56pF C10 1000pF LX AOUT 5 R8 5.6kΩ MAX8597 SS 18 R7 390Ω C3 0.01µF 13 C6 2200pF R6 10kΩ C2B 10µF R2 1.5kΩ C9 8200pF R9 10Ω REFO 8 C11 0.
PAGE 16
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers Setting the Output Voltage Fixed Output Voltage The output voltage is set by a resistor-divider network from the output to GND with FB at the center tap (R4 and R5 in Figure 4).
PAGE 17
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers Output Capacitor The key selection parameters for the output capacitor are the actual capacitance value, the equivalent series resistance (ESR), the equivalent series inductance (ESL), and the voltage-rating requirements, which affect the overall stability, output ripple voltage, and transient response.
PAGE 18
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers where RDH is the high-side MOSFET driver’s average on-resistance (1.25Ω typ) and RGATE is the internal gate resistance of the MOSFET (typically 0.5Ω to 2Ω): the LX voltage waveform can interfere with the current limit. Below is the procedure for selecting the value of the series RC snubber circuit (R14 and C14 in Figure 1): PHSDR = Qgs x VGS x fS x RGATE / (RGATE + RDH) where VGS ~ VVL = 5V.
PAGE 19
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers ILIM ILIM DH R C3 DH RDS(ON) R C3 C RSENSE LX C LX DL DL Figure 5. Adding RC for More Accurate Sensing the MOSFET, it is the inductance from the drain to the source lead. Alternately, to save board space and cost, the RC networks above can be omitted; however, the value of RILIM should be raised to account for the voltage step caused by the inductive divider.
PAGE 20
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers When the output capacitor is comprised of paralleling n number of the same capacitors, then: CO = n x CEACH and RESR = RESR_EACH / n Thus, the resulting fZ_ESR is the same as that of a single capacitor.
PAGE 21
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers GMOD(FC) = GMOD(DC) x (fP_LC)2 / (fZ_ESR x fC) Since the output-capacitor ESR-zero frequency is higher than the LC double-pole frequency but lower than the closed-loop crossover frequency, where the modulator already has -1 slope, the error-amplifier gain must have zero slope at fC so the loop crosses over at the desired -1 slope.
PAGE 22
Applications Information PC Board Layout Guide Careful PC board layout is critical to achieve low switching losses and clean, stable operation. The switching power stage requires particular attention. Follow these guidelines for good PC board layout: 1) Place the high-side MOSFET close to the low-side MOSFET and arrange them in such a way that the drain of the high-side MOSFET and the source of the low-side MOSFET can be tightly decoupled with a 10µF or larger ceramic capacitor.
PAGE 23
Low-Dropout, Wide-Input-Voltage, Step-Down Controllers 24L QFN THIN.EPS PACKAGE OUTLINE, 12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm 21-0139 D 1 2 ______________________________________________________________________________________ 23 MAX8597/MAX8598/MAX8599 Package Information (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.
PAGE 24
MAX8597/MAX8598/MAX8599 Low-Dropout, Wide-Input-Voltage, Step-Down Controllers Package Information (continued) (The package drawing(s) in this data sheet may not reflect the most current specifications. For the latest package outline information, go to www.maxim-ic.com/packages.) PACKAGE OUTLINE, 12, 16, 20, 24, 28L THIN QFN, 4x4x0.8mm 21-0139 D 2 2 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product.