Datasheet

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General Description

The MAX1858 dual, synchronized, step-down controller

generates two outputs from input supplies ranging from

4.75V to 23V. Each output is adjustable from sub-1V to

18V and supports loads of 10A or higher. Input voltage

ripple and total RMS input ripple current are reduced by

synchronized 180° out-of-phase operation.

The switching frequency is adjustable from 100kHz to

600kHz with an external resistor. Alternatively, the con-

troller can be synchronized to an external clock gener-

ated to another MAX1858 or a system clock. One

MAX1858 can be set to generate an in-phase, or 90°

out-of-phase, clock signal for synchronization with addi-

tional controllers. This allows two controllers to operate

either as an interleaved two- or four-phase system with

each output shifted by 90°. The device also features

“first-on/last-off” power sequencing for compatibility

with DSPs, ASICs, and FPGAs, as well as soft-start and

soft-stop to ensure reliable and repeatable power

sequencing.

The MAX1858 eliminates the need for current-sense

resistors by utilizing the low-side MOSFET’s on-resistance

as a current-sense element. This protects the DC-DC

components from damage during output-overload condi-

tions or when output short-circuit faults without requiring a

current-sense resistor. Adjustable foldback current limit

reduces power dissipation during short-circuit condition.

A power-on reset output signals the system when both

outputs reach regulation.

The MAX1858 is available in a 24-pin QSOP package.

An evaluation kit is available to speed designs.

Applications

Network Power Supplies

Telecom Power Supplies

DSP, ASIC, and FPGA Power Supplies

Set-Top Boxes

Broadband Routers

Servers

Features

♦ Two Independent Output Voltages

♦ 180° Out-of-Phase Operation

♦ 90° Out-of-Phase Operation

(Using Two MAX1858s)

♦ Foldback Current Limit

♦ 4.75V to 23V Input Supply Range

♦ 0 to 18V Output-Voltage Range (Up to 10A)

♦ >90% Efficiency

♦ Fixed-Frequency Pulse-Width Modulation (PWM)

Operation

♦ Adjustable 100kHz to 600kHz Switching

Frequency

♦ External SYNC Input

♦ Clock Output for Master/Slave Synchronization

♦ Power-On/-Off Sequencing with Soft-Start and

Soft-Stop

♦ RST Output with 140ms Minimum Delay

♦ Lossless Current Limit (No Sense Resistor)

MAX1858

Dual 180° Out-of-Phase PWM Step-Down

Controller with Power Sequencing and POR

________________________________________________________________ Maxim Integrated Products 1

LX2

DH2

BST2OSC

ILIM2

FB2

COMP2

TOP VIEW

DL2

PGND

DL1CKO

GND

REF

V+

BST1

DH1

LX1

COMP1

FB1

ILIM1

SYNC

QSOP

MAX1858

RST

Pin Configuration

Ordering Information

19-2432; Rev 0; 7/02

EVALUATION KIT MANUAL

AVAILABLE

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.

PART TEMP RANGE PIN-PACKAGE

MAX1858EEG -40°C to +85°C 24 QSOP

Typical Operating Circuit appears at end of data sheet.

Summary of content (21 pages)

PAGE 1
9-2432; Rev 0; 7/02 ANUAL N KIT M IO T A U EVAL BLE AVAILA Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR The MAX1858 dual, synchronized, step-down controller generates two outputs from input supplies ranging from 4.75V to 23V. Each output is adjustable from sub-1V to 18V and supports loads of 10A or higher. Input voltage ripple and total RMS input ripple current are reduced by synchronized 180° out-of-phase operation.
PAGE 2
MAX1858 Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR ABSOLUTE MAXIMUM RATINGS V+ to GND ..............................................................-0.3V to +25V PGND to GND .......................................................-0.3V to +0.3V VL to GND ..................-0.3V to the lower of +6V and (V+ + 0.3V) BST1, BST2 to GND ...............................................-0.3V to +30V LX1 to BST1..............................................................-6V to +0.
PAGE 3
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR (V+ = 12V, EN = ILIM_ = VL, SYNC = GND, IVL = 0mA, PGND = GND, CREF = 0.22µF, CVL = 4.7µF (ceramic), ROSC = 60kΩ, compensation components for COMP_ are from Figure 1, TA = -40°C to +85°C (Note 1), unless otherwise noted.
PAGE 4
Typical Operating Characteristics (Circuit of Figure 1, VIN = 12V, TA = +25°C, unless otherwise noted.) EFFICIENCY vs. LOAD OUTPUT-VOLTAGE ACCURACY vs. LOAD EFFICIENCY (%) OUT1 70 60 50 40 30 20 MAX1858 toc02 OUT2 80 1.0 0.8 OUTPUT VOLTAGE ACCURACY (%) 90 MAX1858 toc01 100 10 0.6 0.4 0.2 OUT2 0 -0.2 OUT1 -0.4 -0.6 -0.8 -1.0 0 0.1 1 10 0 100 5 LOAD (A) 15 10 LOAD (A) VL VOLTAGE ACCURACY vs. LOAD CURRENT SWITCHING FREQUENCY vs. ROSC -0.5 -1.0 -1.
PAGE 5
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR RESET TIMEOUT SOFT-START AND SOFT-STOP WAVEFORM MAX1858 toc07 OUT-OF-PHASE WAVEFORM MAX1858 toc08 5V VEN 0V MAX1858 toc09 VOUT1 20mV/div 5V/div EN 0V 12V VLX1 VOUT2 VOUT2 1V/div 0V 12V VLX2 0V VOUT1 0V 0V 0V VRST 0V VOUT1 1V/div 0V VOUT2 20mV/div 100ms/div 1ms/div EXTERNALLY SYNCHRONIZED SWITCHING WAVEFORM MAX1858 toc10 1µs/div CKO OUTPUT WAVEFORM MAX1858 toc11 5V VSYNC 0V SYNC = GND 5V VCK0 0V 5V VCK0 0V
PAGE 6
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR MAX1858 Pin Description PIN NAME FUNCTION 1 COMP2 Compensation Pin for Regulator 2 (REG2). Compensate REG2’s control loop by connecting a series resistor (RCOMP2) and capacitor (CCOMP2A) to GND in parallel with a second compensation capacitor (CCOMP2B) as shown in Figure 1. 2 3 4 ILIM2 Current-Limit Adjustment for Regulator 2 (REG2). The PGND–LX2 current-limit threshold defaults to 100mV if ILIM2 is connected to VL.
PAGE 7
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR PIN NAME 14 DH1 High-Side Gate Driver Output for Regulator 1 (REG1). DH1 swings from LX1 to BST1. 15 LX1 External Inductor Connection for Regulator 1 (REG1). Connect LX1 to the switched side of the inductor. LX1 serves as the lower supply rail for the DH1 high-side gate driver. 16 BST1 Boost Flying-Capacitor Connection for Regulator 1 (REG1). Connect BST1 to an external ceramic capacitor and diode according to Figure 1.
PAGE 8
MAX1858 Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR VIN 6V - 23V RV+ 4.7Ω 0.1µF V+ CV+ 0.22µF CIN1 2 × 10µF VL 4.7Ω BST1 COUT1 4 × 220µF NH1* L1 1µH R1A 8.06kΩ ** NL1* CIN2 2 × 10µF BST2 CBST2 0.1µF CBST1 0.1µF OUTPUT1 VOUT = 1.8V CVL 4.7µF 4.7Ω DH1 DH2 LX1 LX2 DL1 DL2 NH2* L2 1.2µH ** NL2* OUTPUT2 VOUT = 2.5V R2A 15kΩ COUT2 4 × 220µF PGND FB1 FB2 RCOMP1 5.9kΩ R1B 10kΩ RCOMP2 8.
PAGE 9
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR MAX1858 REF VREF 2V V+ 5V LINEAR REGULATOR MAX1858 GND VL COMP1 BST1 FB1 CONVERTER 1 DH1 SOFT-START DAC SEQUENCING LX1 R Q S Q DL1 PGND OSC SYNC OSCILLATOR CK0 5µA RST ILIM1 RESET EN VREF UVLO AND SHUTDOWN VL - 0.5V VL BST2 DH2 CONVERTER 2 LX2 DL2 COMP2 FB2 ILIM2 Figure 2.
PAGE 10
MAX1858 Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR High-Side Gate-Drive Supply (BST_) Gate-drive voltages for the high-side N-channel switches are generated by the flying-capacitor boost circuits (Figure 3). A boost capacitor (connected from BST_ to LX_) provides power to the high-side MOSFET driver. On startup, the synchronous rectifier (low-side MOSFET) forces LX_ to ground and charges the boost capacitor to 5V.
PAGE 11
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR ply voltage. A 100kΩ resistor works well for most applications. If unused, leave RST grounded or unconnected. Clock Synchronization (SYNC, CKO) VL BST_ 5Ω DH_ LX_ MAX1858 Figure 3. Reducing the Switching-Node Rise Time On the rising edge of EN both controllers enter softstart.
PAGE 12
MAX1858 Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR -IPEAK INDUCTOR CURRENT ILOAD ILIMIT 0 where tON(MIN) is 100ns.
PAGE 13
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR V  -V R _ A = -R _ C  SET OUT   VREF - VSET  where V SET = 1V, V REF = 2V (see the Electrical Characteristics), and VOUT can range from 0 to VSET. Setting the Switching Frequency The controller generates the clock signal by dividing down the internal oscillator or SYNC input signal when driven by an external oscillator, so the switching frequency equals half the oscillator frequency (fSW = fOSC/2).
PAGE 14
MAX1858 Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR Connect ILIM_ to VL for the default 100mV (typ) currentlimit threshold. For an adjustable threshold, connect a resistor (RILIM_) from ILIM_ to GND. The relationship between the current-limit threshold (VITH_) and RILIM_ is: OUT_ REF R_A RILIM _ = FB_ 0.5µA where RILIM_ is in Ω and VITH_ is in V. An RILIM resistance range of 100kΩ to 600kΩ corresponds to a currentlimit threshold of 50mV to 300mV.
PAGE 15
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR  V   L(ILOAD1 - ILOAD2 )2  OUT  + t OFF(MIN)   VINfSW   VSAG =  VIN - VOUT   2COUT VOUT   - t OFF(MIN)   VINfSW   where t OFF(MIN) is the minimum off-time (see the Electrical Characteristics), and fSW is set by ROSC (see the Setting the Switching Frequency section).
PAGE 16
MAX1858 Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR P DH GAIN = +VIN/VRAMP FOR FREQUENCIES LOWER THAN NYQUIST N L W VC M DL LX N FB COMP_ LX RESR RESR = FB COUT COMP_ gM_GROUP COUT gM_GROUP VSET RCOMP_ CCOMP_A L VOUT VSET RCOMP_ CCOMP_A CCOMP_B CCOMP_B Figure 7. Fixed-Frequency Voltage-Mode Control Loop • RCOMP has much higher impedance than CCOMP_A. This is true if, and only if, crossover occurs above fZ_COMP_A.
PAGE 17
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR 50 fLC 40 30 fESR GAIN (dB) 20 10 fZ-COMP_A fCO fSWITCH 0 -10 -20 fCOMP_B IGATE is the average DH driver output current capability determined by: -30 -40 0.001 0.01 0.1  Q + QGD  V I f PNH(SWITCHING) = IN LOAD OSC  GS  2  IGATE  1 FREQUENCY (MHz) Figure 8. Voltage-Mode Loop Analysis All four N-channel MOSFETs must be a logic-level type with guaranteed on-resistance specifications at VGS ≥ 4.5V.
PAGE 18
MAX1858 Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR capacitance must often be added (see the VSAG equation in the Design Procedure section). The absolute point of dropout is when the inductor current ramps down during the minimum off-time (∆IDOWN) as much as it ramps up during the maximum on-time (∆IUP). The ratio h = ∆IUP/∆IDOWN is an indicator of the ability to slew the inductor current higher in response to increased load, and must always be greater than 1.
PAGE 19
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR all these connections on the top layer with wide, copper-filled areas (2oz copper recommended). 2) Mount the controller IC adjacent to the synchronous rectifier MOSFETs (NL _), preferably on the back side in order to keep LX_, PGND_, and DL_ traces short and wide. The DL_ gate trace must be short and wide, measuring 50mils to 100mils wide if the low-side MOSFET is 1in from the controller IC.
PAGE 20
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR MAX1858 Typical Operating Circuit VIN 4.75V TO 23V V+ VL BST1 BST2 DH1 DH2 NH2 OUTPUT1 = 1.8V, 10A OUTPUT2 = 2.
PAGE 21
Dual 180° Out-of-Phase PWM Step-Down Controller with Power Sequencing and POR QSOP.EPS Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.