Datasheet MAX1684, MAX1685 (Maxim) - 10
| Manufacturer | Maxim |
| Description | Low-Noise, 14V Input, 1A, PWM Step-Down Converters |
| Pages / Page | 14 / 10 — Low-Noise, 14V Input, 1A, PWM Step-Down Converters. Synchronous … |
| File Format / Size | PDF / 301 Kb |
| Document Language | English |
Low-Noise, 14V Input, 1A, PWM Step-Down Converters. Synchronous Rectification. PFM Control. MAX1684/MAX1685
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Text Version of Document
Low-Noise, 14V Input, 1A, PWM Step-Down Converters
inductor peak current during the first half of each cycle, For higher VIN at no load, the frequency decreases based on the output error voltage. The MAX1684/ based on the following equation: MAX1685s’ loop gain is relatively low to enable the use f = VOUT / (VIN × 290ns) of a small, low-value output filter capacitor. The 1.4% transient load regulation from 0 to 1A is compensated At medium- to full-load current (>100mA), VIN can by an integrator circuit that lowers DC load regulation increase slightly higher before the frequency decreas- to 0.01% typical. Slope compensation accounts for the es. inductor-current waveform’s down slope during the
Synchronous Rectification
second half of each cycle, and eliminates the inductor- Although the primary rectifier is an external Schottky current staircasing characteristic of current-mode con- diode, a small internal N-channel synchronous rectifier trollers at high duty cycles. allows PWM operation at light loads. During the second
PFM Control
half of each cycle, when the inductor current ramps In low-power mode, the MAX1684/MAX1685 switch below the zero-crossing threshold or when the oscilla- only as needed to service the load. This reduces the tor period ends, the synchronous rectifier turns off. This switching frequency and associated losses in the keeps excess current from flowing backward through P-channel switch, the synchronous rectifier, and the the inductor. Choose an appropriate inductor to limit external inductor. During this PFM operation, a switch- the PWM ripple current through the N-channel FET to
MAX1684/MAX1685
ing cycle initiates when the PFM comparator senses 400mAP-P. that the output voltage has dropped too low. The
Current Limit and Soft-Start
P-channel MOSFET switch turns on and conducts cur- The voltage at ILIM/SS sets the PWM current limit rent to the output-filter capacitor and load. The (ILIM = 1.75A) and the low-power current limit (ILIMLP = MAX1684/MAX1685 then wait until the PFM comparator 380mA). The PWM current limit applies when the senses a low-output voltage again. device is in PWM mode, in synchronized PWM mode, In normal mode at light load (<150mA), the device also or delivering a heavy load in normal mode (Table 1). operates in PFM. The PFM current comparator controls The ILIMLP limit applies when the device is in low-power both entry into PWM mode and the peak switch current mode. An internal 4µA current source pulls ILIM/SS up during PFM operation. Consequently, some jitter is nor- to CVL. To use the maximum current-limit thresholds, mal during transition from PFM to PWM with loads leave ILIM/SS unconnected or connect it to a soft-start around 150mA, and it has no adverse impact on regu- capacitor. Connect an external resistor from ILIM/SS to lation. AGND to adjust the current-limit thresholds.
100% Duty-Cycle Operation
The PWM current-limit threshold is (ILIM ✕ RILIM/SS ✕ As the input voltage drops, the duty cycle increases 4µA) / VREF and is adjustable from 0.5A to 1.75A. until the P-channel MOSFET turns on continuously, The low-power current-limit threshold is equal to (ILIMLP achieving 100% duty cycle. Dropout voltage in 100% ✕ RILIM/SS ✕ 4µA) / VREF and is adjustable from 110mA duty cycle is the output current multiplied by the on- to 380mA. resistance of the internal switch and inductor, approxi- For example, when RILIM/SS is 156kΩ, the PWM current mately 0.35V (IOUT = 1A). limit threshold is 0.88A and the low-power current limit
Very Low Duty-Cycle Operation
threshold is 0.19A. Because of the P-channel minimum on-time and dead- Connect a low-value capacitor from ILIM/SS to AGND time (duration when both switches are off), the to achieve soft-start, limiting inrush current. ILIM/SS MAX1684/MAX1685s’ switching frequency must internally shorts to AGND in shutdown to discharge the decrease in PWM or normal mode to maintain regula- soft-start capacitor. Do not connect ILIM/SS to REF or tion at a very low duty cycle. The total P-channel on- CVL. Determine the soft-start duration by: time and dead-time is 290ns typical. As a result, the tSOFT-START = CILIM/SS(1.25V / 4µA) MAX1684/MAX1685 maintain fixed-frequency regula- tion at no load for V where tSOFT-START is the time from SHDN going high to IN up to 10VOUT and 5VOUT, respectively (see PWM Fixed-Frequency Operation the regulator being able to supply full load current. For Area graph in the Typical Operating Characteristics). example, a 0.1µF capacitor yields 31ms of soft-start.
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inductor peak current during the first half of each cycle, For higher VIN at no load, the frequency decreases based on the output error voltage. The MAX1684/ based on the following equation: MAX1685s’ loop gain is relatively low to enable the use f = VOUT / (VIN × 290ns) of a small, low-value output filter capacitor. The 1.4% transient load regulation from 0 to 1A is compensated At medium- to full-load current (>100mA), VIN can by an integrator circuit that lowers DC load regulation increase slightly higher before the frequency decreas- to 0.01% typical. Slope compensation accounts for the es. inductor-current waveform’s down slope during the
Synchronous Rectification
second half of each cycle, and eliminates the inductor- Although the primary rectifier is an external Schottky current staircasing characteristic of current-mode con- diode, a small internal N-channel synchronous rectifier trollers at high duty cycles. allows PWM operation at light loads. During the second
PFM Control
half of each cycle, when the inductor current ramps In low-power mode, the MAX1684/MAX1685 switch below the zero-crossing threshold or when the oscilla- only as needed to service the load. This reduces the tor period ends, the synchronous rectifier turns off. This switching frequency and associated losses in the keeps excess current from flowing backward through P-channel switch, the synchronous rectifier, and the the inductor. Choose an appropriate inductor to limit external inductor. During this PFM operation, a switch- the PWM ripple current through the N-channel FET to
MAX1684/MAX1685
ing cycle initiates when the PFM comparator senses 400mAP-P. that the output voltage has dropped too low. The
Current Limit and Soft-Start
P-channel MOSFET switch turns on and conducts cur- The voltage at ILIM/SS sets the PWM current limit rent to the output-filter capacitor and load. The (ILIM = 1.75A) and the low-power current limit (ILIMLP = MAX1684/MAX1685 then wait until the PFM comparator 380mA). The PWM current limit applies when the senses a low-output voltage again. device is in PWM mode, in synchronized PWM mode, In normal mode at light load (<150mA), the device also or delivering a heavy load in normal mode (Table 1). operates in PFM. The PFM current comparator controls The ILIMLP limit applies when the device is in low-power both entry into PWM mode and the peak switch current mode. An internal 4µA current source pulls ILIM/SS up during PFM operation. Consequently, some jitter is nor- to CVL. To use the maximum current-limit thresholds, mal during transition from PFM to PWM with loads leave ILIM/SS unconnected or connect it to a soft-start around 150mA, and it has no adverse impact on regu- capacitor. Connect an external resistor from ILIM/SS to lation. AGND to adjust the current-limit thresholds.
100% Duty-Cycle Operation
The PWM current-limit threshold is (ILIM ✕ RILIM/SS ✕ As the input voltage drops, the duty cycle increases 4µA) / VREF and is adjustable from 0.5A to 1.75A. until the P-channel MOSFET turns on continuously, The low-power current-limit threshold is equal to (ILIMLP achieving 100% duty cycle. Dropout voltage in 100% ✕ RILIM/SS ✕ 4µA) / VREF and is adjustable from 110mA duty cycle is the output current multiplied by the on- to 380mA. resistance of the internal switch and inductor, approxi- For example, when RILIM/SS is 156kΩ, the PWM current mately 0.35V (IOUT = 1A). limit threshold is 0.88A and the low-power current limit
Very Low Duty-Cycle Operation
threshold is 0.19A. Because of the P-channel minimum on-time and dead- Connect a low-value capacitor from ILIM/SS to AGND time (duration when both switches are off), the to achieve soft-start, limiting inrush current. ILIM/SS MAX1684/MAX1685s’ switching frequency must internally shorts to AGND in shutdown to discharge the decrease in PWM or normal mode to maintain regula- soft-start capacitor. Do not connect ILIM/SS to REF or tion at a very low duty cycle. The total P-channel on- CVL. Determine the soft-start duration by: time and dead-time is 290ns typical. As a result, the tSOFT-START = CILIM/SS(1.25V / 4µA) MAX1684/MAX1685 maintain fixed-frequency regula- tion at no load for V where tSOFT-START is the time from SHDN going high to IN up to 10VOUT and 5VOUT, respectively (see PWM Fixed-Frequency Operation the regulator being able to supply full load current. For Area graph in the Typical Operating Characteristics). example, a 0.1µF capacitor yields 31ms of soft-start.
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