HP 15c Limited Edition

Power supply current (T_a=25°C, V_battery=3.00V(CR2032×2)) : I_run≈19mA, I_idle≈47μA, I_standby≈4μA

Confirmation of the High power comsumption issue of the HP-15C LE.

Test circuit

The current sense resistance is 5.00Ω ^*1

HP 15C Limited Edition under test - connected to the test circuit.
(DUT : CNA15000DN, Limited Edition Number 16188, 2011-04-15)

Power off → on (10mA / div, 80ms / div) : 42mA peak current.
Power off to on

Power off → on (10mA / div, 1ms / div)
Power off to on

Power off → on (10mA / div, 400us / div)
Power off to on

Power on → off (10mA / div) : 40mA peak current
Power on to off

Pressing [ENTER] key once (10mA / div) : continuous 19mA while pressing a key
Pressing ENTER key once

Executing tight loop program ^*1 (10mA / div) : 19mA while executing a program
Pressing ENTER key once

Note *1 : Current sense resistor : Two thin metal film resistors of 10.0Ω with 0.5% tolerance are used in parallel.

Note *2 : Test program listings

001-42,21,11  LBL A 
002-   43 26  PI
003-   44  0  STO 0
004-      25  TAN
005-   43 35  CLx
006-   45  0  RCL0
007-   22 11  GTO A

Note *3 : Blackout voltage
When the supply voltage goes less than 1.8V, blackout will occur without low battery warning and the memory will be lost.
It will turn on when supply voltage is higher than 2.0V

Workaround for the high current spike

Additional bypassing capacitor of 100uF will reduce the high current spike which may cause "Pr Error".

A multi-layer ceramic capacitor of 100uF/6.3V/X5R (TDK, C3225X5R0J107M) is added at the battery input terminal.

Power off → on (10mA / div, 400us / div) - Current spike had been reduced from 44mA to 18mA by using one C3225X5R0J107M.
TDK, C3225X5R0J107M

Though it's not recommended for new modification, a relatively cheap F1 grade MLCC of 100uF (e.g. Murata, GRM31CF10J107ZE01) can be used instead.

Power off → on (10mA / div, 400us / div) - Current spike had been reduced from 44mA to 21mA by using one GRM31CF10J107ZE01.

Note *4 : recommended MLCCs

TDK, C3225X5R0J107M, 100u/6.3V/X5R/3225 metric
Murata, GRM32ER60J107ME20L, 100u/6.3V/X5R/3225 metric
TDK, C3216X5R0J107M, 100u/6.3V/X5R/3216 metric
TaiyoYuden, JMK316BJ107ML-T, 100u/6.3V/X5R/3216 metric

Speed comparison

HP15C Limited Edition is very fast!
The 15C-LE is 30 times faster than a triple-speed modified 15C.
Though there are some flaws, I like this brand new old fashioned calculator.

HP 15C Limited Edition internal view

PCB - The two batteies are just connected in parallel (Or-ing diodes are omitted).

MCU (Atmel AT91SAM7L128-AU)

Battery holder

Confirmation of the High power comsumption issue of the HP-15C LE.

Workaround for the high current spike

Speed comparison

HP 15C Limited Edition internal view

SEE ALSO