Datasheet MAX756, MAX757 (Maxim) - 6

MAX756/MAX757
3.3V/5V/Adjustable-Output, Step-Up DC-DC Converters
The inductor’s DC resistance significantly affects effi- VIN ciency. For highest efficiency, limit L1’s DC resistance to 0.03Ω or less. See Table 1 for a list of suggested C1 inductor suppliers. 150μF L1 R3 22μH D1 1N5817
Table 1. Component Suppliers
5 8 VOUT LBI LX
PRODUCTION INDUCTORS CAPACITORS
C2 MAX757 6
METHOD
R4 OUT R1 100μF Surface-Mount Sumida AVX 1 2 CD54-220 (22µH) TPS series SHDN FB CoilCraft 3 4 Sprague REF LBO DT3316-223 R2 595D series C3 Coiltronics GND 0.1μF CTX20-1 7 Miniature Sumida Sanyo OS-CON Through-Hole RCH654-220 OS-CON series Figure 1. Standard Application Circuit low-ESR organic semiconductor where V Low-Cost CoilCraft Nichicon IN is the desired threshold of the low-battery detector, R3 and R4 are the input divider resistors at Through-Hole PCH-27-223 PL series LBI, and V low-ESR REF is the internal 1.25V reference. electrolyic Since the LBI current is less than 100nA, large resistor values (typically 10kΩ to 200kΩ) can be used for R3 United Chemi-Con and R4 to minimize loading of the input supply. LXF series When the voltage at LBI is below the internal threshold, AVX USA: (207) 282-5111, FAX (207) 283-1941 LBO sinks current to GND. A pull-up resistor of 10kΩ (800) 282-9975 or more connected from LBO to VOUT can be used CoilCraft USA: (708) 639-6400, FAX (708) 639-1969 when driving CMOS circuits. Any pull-up resistor con- Coiltronics USA: (407) 241-7876, FAX (407) 241-9339 nected to LBO should not be returned to a voltage Collmer source greater than VOUT. When LBI is above the Semiconductor USA: (214) 233-1589 threshold, the LBO output is off. The low-battery com- Motorola USA: (602) 244-3576, FAX (602) 244-4015 parator and reference voltage remain active when the Nichicon USA: (708) 843-7500, FAX (708) 843-2798 MAX756/MAX757 is in shutdown mode. Japan: +81-7-5231-8461, FAX (+81-) 7-5256-4158 If the low-battery comparator is not used, connect LBI Nihon USA: (805) 867-2555, FAX (805) 867-2556 to V Japan: +81-3-3494-7411, FAX (+81-) 3-3494-7414 IN and leave LBO open. Sanyo OS-CON USA: (619) 661-6835
Inductor Selection
Japan: +81-720-70-1005, FAX (+81-720-) 70-1174 The inductors should have a saturation (incremental) Sprague USA: (603) 224-1961, FAX (603) 224-1430 current rating equal to or greater than the peak switch- Sumida USA: (708) 956-0666 current limit, which is 1.2A worst-case. However, it’s Japan: +81-3-3607-5111, FAX (+81-3-) 3607-5428 generally acceptable to bias the inductor into satura- United tion by 20%, although this will reduce the efficiency. Chemi-Con USA: (708) 696-2000, FAX (708) 640-6311 The 22µH inductor shown in the typical applications cir-
Capacitor Selection
cuit is sufficient for most MAX756/MAX757 application A 100µF, 10V surface-mount (SMT) tantalum capacitor circuits. Higher input voltages increase the energy typically provides 50mV output ripple when stepping transferred with each cycle, due to the reduced up from 2V to 5V at 200mA. Smaller capacitors, down input/output differential. Minimize excess ripple due to to 10µF, are acceptable for light loads or in applica- increased energy transfer by reducing the inductor tions that can tolerate higher output ripple. value (10µH suggested). 6 Maxim Integrated