search

myelin / modular-arm-usb

LPC11U14 USB test board by Phil Pearson and Philip Lindsay
0 stars 0 forks source link

Find a better SMD 3v3 regulator #17

Open myelin opened 9 years ago

myelin commented 9 years ago

Microchip MCP1700-3302E/TO can do 250mA w/ 0.178 Vdo, $0.37 That'll let us regulate the battery power also, and drop the diode.

For higher currents: Exar SPX1117M3-L-3-3/TR can do 800 mA w/ 1.1 Vdo, $0.41

follower commented 9 years ago

Available in SOT23.

Product page: MCP1700 -- Low Quiescent Current LDO Datasheet: MCP1700 -- Low Quiescent Current LDO

myelin commented 9 years ago

MCP1700-330* versions:

E/MB - SOT-89-8 http://www.newark.com/microchip/mcp1700t-3302e-mb/ic-ldo-volt-reg-3-3v-250ma-sot/dp/88H9816 E/TO - TO-92 http://www.newark.com/microchip/mcp1700-3302e-to/ic-ldo-volt-reg-3-3v-250ma-to/dp/88H9804 E/TT - SOT-23 http://www.newark.com/microchip/mcp1700t-3302e-tt/ic-ldo-volt-reg-3-3v-250ma-3-sot/dp/88H9817

SOT-23 thermal resistance is about 300 C/W TO-92 ~150 C/W SOT-89 ~50 C/W

dropping 5v -> 3v3 is 1.7v. Pmax = allowable temp rise / thermal resistance = 1.7V * Imax

40C / thermal resistance = 1.7V * Imax

Imax = 40 / ( 1.7 * thermal resistance )

= 470 mA for SOT-89, 156 mA for TO-92, 78 mA for SOT-23

In this case we should be OK with the SOT-23 then. 40 mA * 1.7 V * 300 C/W ~= 20 C temp rise.