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MAX1626 Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX1626 Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 16 page least 100 times FB’s input bias current. Capacitor CR2 is used to compensate the MAX1627 for even switch- ing. Values between 0pF and 330pF work for many applications. See the Stability and MAX1627 Feedback Compensation section for details. Current-Sense-Resistor Selection The current-sense comparator limits the peak switching current to VCS/RSENSE, where RSENSE is the value of the current-sense resistor and VCS is the current-sense threshold. VCS is typically 100mV, but can range from 85mV to 115mV. Minimizing the peak switching current will increase efficiency and reduce the size and cost of external components. However, since available output current is a function of the peak switching current, the peak current limit must not be set too low. Set the peak current limit above 1.3 times the maximum load current by setting the current-sense resistor to: Alternatively, select the current-sense resistor for 5V and 3.3V output applications using the current-sense resistor graphs in Figures 6a and 6b. The current-sense resistor’s power rating should be 20% higher than: Standard wire-wound resistors have an inductance high enough to degrade performance, and are not rec- ommended. Surface-mount (chip) resistors have very little inductance and are well suited for use as current- sense resistors. Power metal-strip resistors feature 1/2W and 1W power dissipation, 1% tolerance, and inductance below 5nH. Resistance values between 10m Ω and 500mΩ are available. Inductor Selection The essential parameters for inductor selection are inductance and current rating. The MAX1626/MAX1627 operate with a wide range of inductance values. In many applications, values between 10µH and 68µH take best advantage of the controller’s high switching frequency. Calculate the minimum inductance value as follows: where 2µs is the minimum on-time. Inductor values between two and six times L(MIN) are recommended. L = V+ - V (MIN) (MAX) OUT () () xs V R CS MIN CS 2 µ R = V R POWER RATING (W) 2 CS CS MAX () R = V 1.3 x I CS CS(MIN) OUT(MAX) 5V/3.3V or Adjustable, 100% Duty-Cycle, High-Efficiency, Step-Down DC-DC Controllers _______________________________________________________________________________________ 9 R3 CR2 R2 FROM OUTPUT TO FB Figure 5. Adjustable-Output Operation Using the MAX1627 4.5 5.5 5.0 6.0 12 10 14 16 INPUT VOLTAGE (V) 3.0 3.5 2.5 2.0 1.5 1.0 0 0.5 VOUT = 5V RSENSE = 0.03Ω RSENSE = 0.04Ω RSENSE = 0.05Ω RSENSE = 0.1Ω Figure 6a. MAX1626 5V-Operation Current-Sense Resistor Graph 3.0 4.0 3.5 4.5 12 10 14 16 INPUT VOLTAGE (V) 3.0 3.5 2.5 2.0 1.5 1.0 0 0.5 VOUT = 3.3V RSENSE = 0.03Ω RSENSE = 0.04Ω RSENSE = 0.05Ω RSENSE = 0.1Ω Figure 6b. MAX1626 3.3V-Operation Current-Sense Resistor Graph |
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