Just a quick review of pyrocircuits used in altimeters.
There are two types of pyrocircuits currently used in hobby rocketry altimeters: 1.) Switched DC pyro circuits. (conventional); and 2.) CD (capacitor discharge) circuits.
The power developed by a pyrocircuit is the product of the supply voltage and the current drawn by the igniter circuit.
P = I*V = Vbattery^2/Rtotal = Rtotal*I^2 where I is current, Vbattery is battery voltage, and Rtotal is the total series resistance of Rigniter, Rtransistor, Rwiring and Rbattery.
The power delivered to the igniter is P = Rigniter*I^2.
With a switched DC pyrocircuit, current sourced from a battery is switched electronically to activate a pyrotechnic device. At a minimum, the battery, switch and wiring must be able to source, at a minimum, the rated all-fire current of the e-match or pyrotechnic device to be activated (This is only known if if it is measured or supplied by the manufacturer.). Ideally you should be able to source 2-3 times this value to insure that if the pyro device is an e-match of igniter, it activates it's pyrogen in only a few milliseconds.
There are several reasons to use this type of circuit. If you use an unboosted (pyrogen free) resistance wire to activate a Pyrodex(R) ejection charge (or BP charge for that matter), you may need to heat the wire of several tenths of a second to ignite the charge. Or if you are activating a high current device that requires a lot of energy (E = P*t where E is the energy in Joules and P is the power in watts and t is time in seconds.).
To deliver high currents for long periods of time (tenths of seconds) your pyrocircuit requires a battery with a low internal resistance (high current rating), a low resistance electronic switch (transistor, FET or MOSFET), and low resistance wiring. The electronic switch must be rated for the short circuit current of the pyrocircuit, otherwise the maximum current that can pass through the switch must be limited with a series resistor.
The disadvantage of this pyrocircuit is that you need a large battery to supply the current. You also need to protect the microprocessor from a brownout when the pyrocircuit is activated. This can be achieved by using separate batteries for the microprocessor and the pyrocircuit, or by buffering the microprocessor from voltage dropouts with a current limiting resistor feeding an energy storage buffer capacitor.
The CD pyrocircuit was developed (and is used in many other applications) to delivery high current (high power) pulsed power for "short" time periods from devices employing small batteries. The PerfectFlite MAWD altimeter is an example of this applied to hobby rocketry.
http://www.perfectflite.com/Downloads/MAWDManual.pdf
In the PF MAWD, a pyrocircuit consisting of 4700 uF capacitor and a low on-resistance MOSFET can deliver up to 27 amps of current for a short time. The capacitor stores 0.19 Joules which is more than enough to activate a properly selected commercial e-matches. For example the on-resistance of the MAWD pyrocircuit is 0.33 ohms. (R = V/I = 9/27 = 0.33), and a common e-match resistance is 1.6 ohms. The old DaveyFire N28F e-match had a 1.6 ohm resistance, a 1 amp all-fire current, and a 3x all-fire activation time of ~2 milliseconds. This is an all-energy of only E t*R*I^2 = 0.002 * 1.6 * 3^2 = 0.029 Joules. The Rtotal of the MAWD pyrocircuit is ~2 ohms, so the igniter will draw an initial 4.5 amps from a capacitor charged to 9 volts.
The discharge current of a capacitor follows an exponential decay equation.
http://www.antonine-education.co.uk/physics_a2/module_4/Topic_7/Topic_7.htm
I = Io*e(-t/RC) = 4.5 * e(-t/0.0075) where RC = 1.6*0.0047 = 0.0075 seconds. In 0.002 seconds, the current will have dropped to ~3.5 amps, so in 0.002 seconds, the CD circuit has delivered an average of 4 amps or ~0.05 Joules into the igniter which is more than required to activate the pyrogen on the e-match. Since the energy is stored and delivered by the capacitor and not directly from the battery, the current capability of the battery can be much lower. The microprocessor in the MAWD draws only about 0.008 amps and can be powered with only 5 or 6 type SR-44/357 Silver Oxide cells in series. These tiny button cells can supply 0.1 amps on a short, but are rated for ~0.020 amps of continuous draw. 6 AgO batteries in series supply 9 volts. If the charge current is limited to 0.020 amps, the 0.19 Joules of energy stored in the capacitor can be recharged in 1 second! 6 silver oxide cells weigh a fraction of the weight of a 9 volt alkaline battery, but will provide the equivalent function if a capacitive discharge circuit is employed.
Hope this helps.
Bob