Datasheet

ManualsBrandsMAXIM INTEGRATED ManualsPMICsPMIC - voltage regulators - DC DC switch controllers QSOP 24

General Description

The MAX1875/MAX1876 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 MAX1875/MAX1876 or a system clock.

One MAX1875/MAX1876 can be set to generate an in-

phase, or 90° out-of-phase, clock signal for synchro-

nization with additional controllers. This allows two

controllers to operate either as an interleaved two- or

four-phase system with each output shifted by 90°.

These devices also feature soft-start and soft-stop.

The MAX1875/MAX1876 eliminate 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 conditions or when output short-circuit faults

without requiring a current-sense resistor. Adjustable

foldback current limit reduces power dissipation during

short-circuit conditions. The MAX1876 includes a

power-on reset output to signal the system when both

outputs reach regulation.

The MAX1875/MAX1876 are 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 MAX1875/MAX1876s)

♦ 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

♦ Soft-Start and Soft-Stop

♦ RST Output with 140ms Minimum Delay (MAX1876)

♦ Lossless Current Limit (No Sense Resistor)

MAX1875/MAX1876

Dual 180° Out-of-Phase PWM Step-

Down Controllers with 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

MAX1875/

MAX1876

RST

(MAX1876 ONLY)

Pin Configuration

Ordering Information

19-2431; Rev 0; 7/02

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

MAX1875EEG -40°C to +85°C 24 QSOP

MAX1876EEG -40°C to +85°C 24 QSOP

Typical Operating Circuit appears at end of data sheet.

Summary of content (21 pages)

PAGE 1
9-2431; Rev 0; 7/02 KIT ATION EVALU E L B A IL AVA Dual 180° Out-of-Phase PWM StepDown Controllers with POR The MAX1875/MAX1876 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
MAX1875/MAX1876 Dual 180° Out-of-Phase PWM StepDown Controllers with 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 StepDown Controllers with 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 MAX1875 toc02 OUT2 80 1.0 0.8 OUTPUT VOLTAGE ACCURACY (%) 90 MAX1875 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) SWITCHING FREQUENCY vs. ROSC -0.5 -1.0 -1.5 MAX1875 toc04 0 600 SWITCHING FREQUENCY (kHz) MAX1875 toc03 0.
PAGE 5
Dual 180° Out-of-Phase PWM StepDown Controllers with POR SOFT-START AND SOFT-STOP WAVEFORM RESET TIMEOUT (MAX1876 ONLY) OUT-OF-PHASE WAVEFORM MAX1875 toc08 MAX1875 toc07 MAX1875 toc09 5V/div 5V EN 0V VOUT1 20mV/div EN 0 12V VLX1 VOUT2 VOUT2 1V/div 0V VOUT2 1V/div 100ms/div 1ms/div MAX1875 toc10 0V VRST 0V 0V EXTERNALLY SYNCHRONIZED SWITCHING WAVEFORM 0V 12V VLX2 0V VOUT1 0V VOUT2 20mV/div 1µs/div CKO OUTPUT WAVEFORM CKO OUTPUT WAVEFORM MAX1875 toc11 5V VSYNC 0V MAX1875 toc12 SYNC
PAGE 6
Dual 180° Out-of-Phase PWM StepDown Controllers with POR MAX1875/MAX1876 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 StepDown Controllers with POR PIN NAME 14 DH1 High-Side Gate Driver Output for Regulator 1 (REG1). DH1 swings from LX1 to BST1. FUNCTION 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
MAX1875/MAX1876 Dual 180° Out-of-Phase PWM StepDown Controllers with POR VIN 6V - 23V RV+ 4.7Ω 0.1µF V+ CV+ 0.22µF CIN1 2 × 10µF VL 4.7Ω COUT1 4 × 220µF NH1* L1 1µH R1A 8.06kΩ ** NL1* CIN2 2 × 10µF BST2 BST1 CBST2 0.1µF CBST1 0.1µF OUTPUT1 VOUT = 1.8V CVL 4.7µF 4.7Ω DH1 DH2 LX1 LX2 DL1 DL2 NH2* OUTPUT2 VOUT = 2.5V L2 1.2µH ** NL2* R2A 15kΩ COUT2 4 × 220µF PGND FB1 FB2 RCOMP1 5.9kΩ R1B 10kΩ RCOMP2 8.
PAGE 9
Dual 180° Out-of-Phase PWM StepDown Controllers with POR MAX1875/MAX1876 REF VREF 2.0V V+ 5V LINEAR REGULATOR MAX1875 MAX1876 GND VL COMP1 BST1 FB1 CONVERTER #1 DH1 LX1 SOFT-START DAC R Q S Q DL1 PGND OSC SYNC OSCILLATOR CK0 5µA RST (MAX1876 ONLY) ILIM1 RESET EN VREF UVLO AND SHUTDOWN VL - 0.5V VL BST2 DH2 CONVERTER 2 LX2 DL2 COMP2 FB2 ILIM2 Figure 2.
PAGE 10
MAX1875/MAX1876 Dual 180° Out-of-Phase PWM StepDown Controllers with 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 StepDown Controllers with POR VL BST_ 5Ω DH_ LX_ MAX1875 Figure 3. Reducing the Switching-Node Rise Time On the rising edge of EN both controllers enter softstart. Soft-start gradually ramps up to the reference voltage seen by the error amplifier in order to control the outputs’ rate of rise and reduce input surge currents during startup. The soft-start period is 1024 clock cycles (1024/f SW ), and the internal soft-start DAC ramps up the voltage in 64 steps.
PAGE 12
-IPEAK ILOAD INDUCTOR CURRENT MAX1875/MAX1876 Dual 180° Out-of-Phase PWM StepDown Controllers with POR ILIMIT 0 the sum of the resistances in the charging path, including high-side switch, inductor, and PC board resistances. Setting the Output Voltage For 1V or greater output voltages, set the MAX1875/ MAX1876 output voltage by connecting a voltagedivider from the output to FB_ to GND (Figure 6). Select R_B (FB_ to GND resistor) to between 1kΩ and 10kΩ.
PAGE 13
Dual 180° Out-of-Phase PWM StepDown Controllers with POR ROSC = 6 × 109 Ω - Hz S fSW where fSW is in Hz, fOSC is in Hz, and ROSC is in Ω. For example, a 600kHz switching frequency is set with ROSC = 10kΩ. Higher frequencies allow designs with lower inductor values and less output capacitance. Consequently, peak currents and I2R losses are lower at higher switching frequencies, but core losses, gatecharge currents, and switching losses increase.
PAGE 14
MAX1875/MAX1876 Dual 180° Out-of-Phase PWM StepDown Controllers with POR An RILIM resistance range of 100kΩ to 600kΩ corresponds to a current-limit threshold of 50mV to 300mV. When adjusting the current limit, 1% tolerance resistors minimize error in the current-limit threshold. For foldback current limit, a resistor (RFBI) is added from ILIM pin to output.
PAGE 15
Dual 180° Out-of-Phase PWM StepDown Controllers with 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). Compensation Each voltage-mode controller section employs a transconductance error amplifier whose output is the compensation point of the control loop.
PAGE 16
MAX1875/MAX1876 Dual 180° Out-of-Phase PWM StepDown Controllers with POR GAIN = +VIN/VRAMP FOR FREQUENCIES LOWER THAN NYQUIST DH P N L W VC LX DL M N FB COMP_ LX RESR RESR = COUT FB 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 A final pole is added using C COMP_B to reduce the gain and attenuate noise after crossover.
PAGE 17
Dual 180° Out-of-Phase PWM StepDown Controllers with POR 50 fLC 40 30 fESR GAIN (dB) 20 10 fZ-COMP_A fCO fSWITCH 0 -10 -20 fCOMP_B -30 -40 0.001 0.01 0.1 1 FREQUENCY (MHz) Figure 8. Voltage-Mode Loop Analysis • Minimum threshold voltage (VTH(MIN)) • Total gate charge (Qg) • Reverse transfer capacitance (CRSS) • Power dissipation All four N-channel MOSFETs must be a logic-level type with guaranteed on-resistance specifications at VGS ≥ 4.5V.
PAGE 18
MAX1875/MAX1876 Dual 180° Out-of-Phase PWM StepDown Controllers with POR the switching frequency and minimum off-time specifications. This error is more significant at higher frequencies. Also, keep in mind that transient response performance of buck regulators operated close to dropout is poor, and bulk output capacitance must often be added (see the VSAG equation in the Design Procedure section).
PAGE 19
Dual 180° Out-of-Phase PWM StepDown Controllers with POR Layout Procedure 1) Place the power components first, with ground terminals adjacent (NL _ source, CIN_, and COUT_). Make 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.
PAGE 20
Dual 180° Out-of-Phase PWM StepDown Controllers with POR MAX1875/MAX1876 Typical Operating Circuit VIN 4.75V TO 23V V+ VL BST1 BST2 DH1 DH2 LX1 LX2 DL1 DL2 NH2 OUTPUT1 = 1.8V, 10A OUTPUT2 = 2.
PAGE 21
Dual 180° Out-of-Phase PWM StepDown Controllers with 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.