www.latticesemi.com 1 an6050_01 controlling power mosfets using the ISPPAC-POWR604 april 2003 application note an6050 introduction the isppac ? -powr604 is a member of lattice semiconductor?s family of mixed-signal power-supply manager ics. the ISPPAC-POWR604 is useful in cost-sensitive applications requiring simple control logic and which do not require the high-side mosfet drive capabilities of the isppac-powr1208. table 1 summarizes and compares the total resources available on each of these devices. where the full resources of the isppac-powr1208 are not required, the ISPPAC-POWR604 can often be an economical alternative. ta b le 1. resource comparison between ISPPAC-POWR604 and isppac-powr1208 in applications in which power mosfets need to be switched, it is often possible to use the ISPPAC-POWR604 despite its absence of charge-pumped fet drivers through the use of some simple and inexpensive external cir- cuitry. this application note will describe several ways to interface the ISPPAC-POWR604 to external mosfet s witches. driving an n-channel mosfet n-channel mosfets are frequently used as high-side switches because of their low on-resistances and low cost. to completely turn on an n-channel mosfet, however, requires that a positive gate-to-source voltage typically r anging from 2v to 10v be applied to the device. when the mosfet is used as a high-side switch, this means that the voltage at the gate terminal will need to be raised signi cantly above the drain terminal?s voltage. because the ISPPAC-POWR604?s outputs are rated to a maximum voltage of 6v, this will often require the use of an external driver circuit, such as the one shown in figure 1. figure 1. driving an n-channel high-side switch if high voltage supply is available feature ISPPAC-POWR604 isppac-powr1208 analog inputs/comparators 6 12 digital inputs 4 4 logic macrocells 8 16 timers 2 4 logic outputs 4 8 hi-v mosfet-driver outputs 0 4 v+ (5 to 12v) ISPPAC-POWR604 out5 r 1 4.7k r 2 4.7k r 3 51 f rom supply to load q 1 mmbt3904lt1 n-channel pow er fet z 1 (optional)
controlling power mosfets using lattice semiconductor the ISPPAC-POWR604 2 in this circuit an auxiliary high-voltage source (v+) is required to bias the mosfet. the external transistor driver is used for two reasons. the rst is to allow operation at higher voltages than the ISPPAC-POWR604?s outputs will withstand (6v). the second reason for using this circuit is that if the ISPPAC-POWR604 is programmed to reset and start up in the high state, this circuit will ensure that the mosfet is not inadvertently turned on if the v+ supply energizes before the supply for the ISPPAC-POWR604. to turn the mosfet on requires that the isppac- powr604?s output go low. zener diode z 1 is provided to ensure that the mosfet?s gate voltage does not exceed its maximum ratings. this situation can occur if a mosfet with a low gate-source breakdown voltage is speci ed, and the circuit is driven from a high voltage +v supply. in many cases, the addition of the zener diode will not be necessary. resistor r 3 should be put in series with the mosfet gate close to the device to help reduce the chance of spurious rf oscil- lation. the preferred value of this gate resistor will be a function of the particular mosfet used and the circuitry to which it is connected. driving a p-channel mosfet when a high voltage supply is not readily available to provide gate drive for an n-channel mosfet, it may be pos- sible to use a p-channel device. this is because when a p-channel mosfet is used to switch a positive power rail, the gate must be pulled negative in relation to the source to turn it on. if the power rail is at a voltage greater than the mosfet?s turn-on voltage, this can be accomplished by pulling the gate to ground. in cases where the positive supply being switched is less than maximum output voltage of the ISPPAC-POWR604, this can be accomplished by driving the mosfet directly from the ISPPAC-POWR604?s output, where a low output signal results in turning the mosfet on. as in the case of the previous circuit example, this will result in the circuit powering up in a ?safe? state if the ISPPAC-POWR604 output is programmed to power-up high. a pull-up resistor (r 1 ) is needed between the mosfet?s gate and source to ensure that the device turns off when the ISPPAC-POWR604?s output goes high. figure 2. driving a p-channel high-side switch one caveat that applies to this circuit is that it is only effective in switching supply rails which are at a voltage higher than the mosfet?s on voltage. for example, if one were to try switching a 1.2v rail with this circuit, it would be e xtremely dif cult, if not impossible, to obtain a p-channel power mosfet which could be completely switched on with only -1.2v of gate-source drive. for this reason, this circuit solution is only presently practical for use with power rails ranging from ~2.5v to 6v. a simple external charge-pump driver in systems where the use of a p-channel mosfet switch would be unsuitable and an external high-voltage source is not available for driving an n-channel device, it may be possible to generate a small amount of current at a higher v oltage with the addition of some external circuitry. this external ?step-up? circuit needs to be small, inexpensive, low-power, and not generate excessive noise. while a dc-to-dc converter could be used to provide the necessary ISPPAC-POWR604 out5 r 1 4.7k r 2 51 f rom supply (<6v) to load p-channel pow er fet
controlling power mosfets using lattice semiconductor the ISPPAC-POWR604 3 v oltage and can be implemented with a single ic and a few external components (or simply purchased as a n- ished module) this approach tends to be expensive. because only a miniscule amount of current is needed to charge a mosfet?s gate, however, one alternative solution is to use a capacitive voltage multiplier. capacitive volt- age multipliers are simple, inexpensive, electrically quiet, and can be operated from small amounts of power. figure 3 shows an example of a 3x voltage multiplier, which is driven by the ISPPAC-POWR604?s clock output, and generates an 8v to 9v dc gate drive voltage. figure 3. external charge pump driver for n-channel high-side switch adding a pull-up resistor to the ISPPAC-POWR604?s clock output results in a 3.3v peak-to-peak square wave out- put at the device?s internal clock frequency, approximately 250 khz. on each low-to-high cycle of the clock, more charge is forced into capacitors c 4 through c 6, gr adually raising their voltages, with the highest nal voltage appearing on c 6. because of the voltage drops associated with the diodes, the output voltage is not quite 3 times the input voltage (9.9v), but will typically range from 8v to 9v. out5 is used to control the mosfet?s gate. when out5 is low, transistor q 1 is switched off, and the gate will be free to rise to the voltage multipliers maximum output, and will turn on the mosfet. when out5 goes high, tran- sistor q 1 switches on and will short the mosfet gate terminal to ground, turning off the mosfet. because the charge pump is actually powered from the ISPPAC-POWR604?s clock output, this circuit guarantees safe start-up conditions for the mosfet. this is because the ISPPAC-POWR604 will be at least well into power-on- reset by the time enough gate voltage is available to turn on the mosfet. because the above circuit is suitable for controlling a single external mosfet, if one wants to control multiple mosfets, it must be duplicated for each device being controlled. driving multiple copies of this circuit from a sin- gle ISPPAC-POWR604 clock circuit is not recommended because of potential adverse loading effects on the clock output. in cases where one needs to provide high-voltage drive to multiple mosfets, a better design choice would be to use the isppac-powr1208, which provides four internal high-voltage mosfet drivers. related literature ? ISPPAC-POWR604 data sheet ? an6048 - using power mosfets with the isppac-powr1208 te c hnical support assistance hotline: 1-800-lattice (domestic) 1-408-826-6002 (international) e-mail: isppacs@latticesemi.com internet: www .latticesemi.com +3.3v ISPPAC-POWR604 out5 clk c 1 1000pf c 2 1000pf c 3 1000pf c 4 1000pf c 5 1000pf c 6 0.01uf r 1 2.2k r 2 2.2k d 1a d 1b bav99 d 2a d 2b bav99 d 3a d 3b bav99 q 1 mmbt3904lt1 high-side pow er mosfet r 3 51
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