EVALUATION KIT AVAILABLE MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current General Description The MAX16955 is a current-mode, synchronous PWM step-down controller designed to operate with input voltages from 3.5V to 36V while using only 50μA of quiescent current at no load. The switching frequency is adjustable from 220kHz to 1MHz by an external resistor and can be synchronized to an external clock up to 1.1MHz.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current ABSOLUTE MAXIMUM RATINGS SUP and EN to SGND ............................................-0.3V to +42V LX to PGND ..............................................................-1V to +42V BST to LX .................................................................-0.3V to +6V BIAS, FB, PGOOD, FSYNC to SGND .......................-0.3V to +6V DH to LX ...................................................................-0.3V to +6V DL to PGND ...
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current ELECTRICAL CHARACTERISTICS (continued) (VSUP = VEN = 14V, CIN = 10μF, COUT = 94μF, CBIAS = 2.2μF, CBST = 0.1μF, RFOSC = 76.8kΩ, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.) (Note 2) PARAMETER SYMBOL FB Line Regulation CONDITIONS MIN TYP MAX UNITS 0.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current ELECTRICAL CHARACTERISTICS (continued) (VSUP = VEN = 14V, CIN = 10μF, COUT = 94μF, CBIAS = 2.2μF, CBST = 0.1μF, RFOSC = 76.8kΩ, TA = TJ = -40°C to +125°C, unless otherwise noted. Typical values are at TA = +25°C.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Typical Operating Characteristics (VSUP = VEN = 14V, CIN = 10μF, COUT = 94μF, CBIAS = 2.2μF, CBST = 0.1μF, RFOSC = 66.5kΩ, fOSC = 468kHz, VFB = VBIAS, VOUT = 5V, TA = +25°C, unless otherwise noted.) VOUT = 3.3V STARTUP RESPONSE (SKIP MODE) VOUT = 5V STARTUP RESPONSE (SKIP MODE) MAX16955 toc01 MAX16955 toc02 10V/div 10V/div VOUT 2V/div VOUT 2V/div IOUT 2A/div IOUT 2A/div 5V/div VPGOOD 5V/div VPGOOD 2ms SUPPLY CURRENT vs.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VEN = 14V, CIN = 10μF, COUT = 94μF, CBIAS = 2.2μF, CBST = 0.1μF, RFOSC = 66.5kΩ, fOSC = 468kHz, VFB = VBIAS, VOUT = 5V, TA = +25°C, unless otherwise noted.) 0 TO 4A LOAD-TRANSIENT RESPONSE, VOUT = 5V, PWM MODE 0 TO 4A LOAD-TRANSIENT RESPONSE, VOUT = 3.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VEN = 14V, CIN = 10μF, COUT = 94μF, CBIAS = 2.2μF, CBST = 0.1μF, RFOSC = 66.5kΩ, fOSC = 468kHz, VFB = VBIAS, VOUT = 5V, TA = +25°C, unless otherwise noted.) SHORT-CIRCUIT RESPONSE, VOUT = 3.3V SHORT-CIRCUIT RESPONSE, VOUT = 5V MAX16955 toc17 MAX16955 toc16 5V 3.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Typical Operating Characteristics (continued) (VSUP = VEN = 14V, CIN = 10μF, COUT = 94μF, CBIAS = 2.2μF, CBST = 0.1μF, RFOSC = 66.5kΩ, fOSC = 468kHz, VFB = VBIAS, VOUT = 5V, TA = +25°C, unless otherwise noted.) 14 12 10 8 6 4 1.0 10.50 10.45 10.40 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 10.35 -0.8 2 -1.0 10.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Pin Configuration TOP VIEW SUP 1 EN 2 FOSC 3 + 16 BST 15 DH 14 LX MAX16955 FSYNC 4 13 BIAS SGND 5 12 DL COMP 6 11 PGND FB 7 10 PGOOD 9 OUT EP CS 8 TSSOP Pin Description PIN NAME FUNCTION 1 SUP Input Supply Voltage. SUP is the input voltage to the internal linear regulator. Bypass SUP to PGND with a 1μF minimum value ceramic capacitor. Connect BIAS and SUP to a 5V rail, if available.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Pin Description (continued) 10 PIN NAME FUNCTION 9 OUT 10 PGOOD 11 PGND 12 DL 13 BIAS 14 LX External Inductor Connection. Connect LX to the switched side of the inductor. LX serves as the lower supply rail for the DH high-side gate driver. 15 DH High-Side Gate-Driver Output. DH swings from LX to BST.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Functional Diagram SUP EN BST EN LDO BST SWITCH MAX16955 UG DH SGND LX BIAS BUCK CONTROLLER PGOOD REF BIAS FB LG DL PWM EAFB COMP EA REF PGND ILIM OSC FOSC CLK CS ZX FSYNC MODE SYNC CS MODE OUT FBI SGND Maxim Integrated 11
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Detailed Description The MAX16955 is a current-mode, synchronous PWM buck controller designed to drive logic-level MOSFETs. The device tolerates a wide 3.5V to 42V input voltage range and generates an adjustable 1V to 10V or fixed 5V output voltage. This device can operate in dropout mode, making it ideal for automotive and industrial applications with undervoltage transients.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Error Detection and Fault Behavior Several error-detection mechanisms prevent damage to the MAX16955 and the application circuit: • • • • • Overcurrent protection Output overvoltage protection Undervoltage lockout at BIAS Power-good detection of the output voltage Overtemperature protection of the IC Overcurrent Protection The MAX16955 provides cycle-by-cycle current limiting as long as the FB voltage is greater than 0.7V (i.e.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Forced Fixed-Frequency PWM Mode The low-noise forced fixed-frequency PWM mode (FSYNC connected to BIAS or an external clock) disables the zero-crossing comparator, which controls the low-side switch on-time. This forces the low-side gatedriver waveform to constantly be the complement of the high-side gate-drive waveform. The inductor current reverses at light loads while DH maintains a duty factor of VOUT/VSUP.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current tON(SKIP) = VOUT VSUPfSW INDUCTOR CURRENT IPK IL ILOAD = IPK/2 0 ON-TIME TIME Figure 1. Pulse-Skipping/Discontinuous Crossover Point Synchronous rectification reduces conduction losses in the rectifier by replacing the normal low-side Schottky catch diode with a low-resistance MOSFET switch. The internal pulldown transistor that drives DL low is robust, with a 1.6Ω (typ) on-resistance.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Setting the Output Voltage Connect FB to BIAS to enable the fixed step-down controller output voltage (5V), set by a preset, internal resistive voltage-divider connected between the output (OUT) and SGND. To achieve other output voltages between 1V to 10V, connect a resistive divider from OUT to FB to SGND (Figure 2). Select RFB2 (FB to SGND resistor) less than or equal to 100kΩ.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Table 1. Inductor Size Comparison INDUCTOR SIZE SMALLER LARGER Lower price Smaller ripple Smaller form factor Higher efficiency Faster load response Larger fixed-frequency range in skip mode The exact inductor value is not critical and can be adjusted to make trade-offs among size, cost, efficiency, and transient response requirements. Table 1 shows a comparison between small and large inductor sizes.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current INPUT (VIN) CIN MAX16955 DH NH L RSENSE LX DL NL COUT DL GND CS OUT a) OUTPUT SERIES RESISTOR SENSING INPUT (VIN) CIN MAX16955 DH NH INDUCTOR L RDCR LX DL NL DL R1 COUT R2 RCSHL = GND RDCR = CEQ CS ( ) R2 RDCR R1 + R2 L CEQ [ 1 1 + R1 R2 ] OUT b) LOSSLESS INDUCTOR SENSING Figure 4.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current where: ΔIL = ( VSUP − VOUT ) × VOUT VSUP × fSW × L longer a problem (see the VSAG and VSOAR equations in the Transient Response section). However, low-value filter capacitors typically have high-ESR zeros that can affect the overall stability.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current The basic regulator loop is modeled as a power modulator, output feedback divider, and an error amplifier. The power modulator has a DC gain set by g mc × RLOAD, with a pole and zero pair set by RLOAD, the output capacitor (COUT), and its ESR. The following equations determine the approximate value for the gain of the power modulator (GAINMOD(dc)), neglecting the effect of the ramp stabilization.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current If fzMOD is less than 5 x fC, add a second capacitor, CF, from COMP to SGND and set the compensation pole formed by R C and C F (f pEA ) at the f zMOD . Calculate the value of CF as follows: CF = 1 2π × fzMOD × RC As the load current decreases, the modulator pole also decreases; however, the modulator gain increases accordingly and the crossover frequency remains the same.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current During recharge, the internal bootstrap switch acts as a resistor, resulting in an RC circuit with the associated time constants. Two τs (time constants) are necessary to charge from 90% to 99%. The maximum allowable capacitance is, therefore: CBST(MAX) = 2 × RBST(MAX) QG VBIAS(MIN) − VTH(TYP) − 2V Should the minimum value still be too large to be recharged sufficiently, a parallel bootstrap Schottky diode may be necessary.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Applications Information Place the controller IC adjacent to the synchronous rectifier MOSFET (NL) and keep the connections for LX, PGND, DH, and DL short and wide. Use multiple small vias to route these signals from the top to the bottom side if these signals need to be routed in the bottom layer. The gate current traces must be short and wide, measuring 50 mils to 100 mils wide if the low-side MOSFET is 1in from the controller IC.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current VBAT 5.5V TO 28V C1 C2 C3 1 SUP R4 2 VEN DH EN BST R7 10 *VBIAS PGOOD MAX16955 D1 6 R6 LX DL COMP C9 PGND C8 4 R5 3 5 FSYNC CS 15 N1-B 16 L1 C4 14 12 N1-A 11 8 R3 FOSC SGND OUT BIAS 13 9 VOUT 1.2V/5A FB R1 7 C5 C6 C7 R2 *CONNECT FSYNC TO BIAS FOR FIXED-FREQUENCY PWM MODE. CONNECT FSYNC TO SGND FOR SKIP MODE. Figure 7. Typical Operating Circuit for Adjustable Output Voltage, VOUT = 1.
MAX16955 36V, 1MHz Step-Down Controller with Low Operating Current Revision History REVISION NUMBER REVISION DATE 0 3/11 Initial release 1 9/12 Updated limiting output range to 10V 2 1/13 Added MAX16955AUE+ to Ordering Information 1 3 12/13 Updated Transient Response section 17 4 7/14 DESCRIPTION PAGES CHANGED — Updated Electrical Characteristics for DH and DL gate-driver on-resistance, TOCs 11, 12, 14, 16, 17, 27–30, pins 12 and 15 in Pin Description; and updated the Fixed 5V Linear Reg
