13 Replies Latest reply: Apr 26, 2013 6:24 AM by ndtgenieros RSS

MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)

ndtgenieros Level 4



I´m currently designing a 5 cell pack Li-ion charger with LTC4008 chip (discussion posted in Sep 2011 in this thread)

In page 15 of LTC4008 datasheet you can find the topic about the MOSFET selection.


I´m certainly lost in the proper selection. I know that logic-level MOSFETS must be used (gate voltage around (5.6V-6V), but is the first time for me designing with mosfets.


One important point is the output voltage of the carger, datasheet says that if Vout>20V then the RDSon should be little more high for reducing the ripple...


The charge voltage will be 20.5V and maximum Output Current 1.5A


I was thinking on use the mosfets that appear in the schematics of the datasheet, but has anyone any recommendation?, I mean, do I have to go deep in the mosfets area or can I use one recommended by anyone who has be involved in similar scenarios?


The only important thing in this design is the proper selection for minimum ripple current, at cost of something (as always). Efficiency for example is not a problem cause it´s a battery charger (I prefer to lost energy efficency if that means reduce ripple current in the output). I´m thinking to use a slightly high Inductor (around 40 microH) and two low ESR tantalum capacitors "surge robust" in parallel in the output of the mosfets.


Any comments are appreciated


PS: I need reference values. (What means high ... what means low... 0.001 - 0.01 ? (in mosfet parameters)

  • 1. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    shabaz Level 15

    I'd go with the ones used in their eval board here. 20.5V is borderline higher than 20V so determining the effect of RDSon without any further details may need to be experimental.

    The MOSFETs in their eval board look appropriate for your charge voltage, assuming the input is less than 25V or so (to not operate against max thresholds). The gate voltage of both the MOSFETs is low,

    and the datasheet specifies the p-channel gate voltage range (VCLP-VTGATE) to be well below -20V for the on condition (and below 50mV when off, so the MOSFETs will certainly switch off) and you're safe there too.

    I havn't confirmed power dissipation you should expect, but that should be an easy calculation, based on the RDSon value, and is likely fine.

  • 2. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4

    The same mosfets appear in LTC4008 datasheet (page 20)...


    Q1: Si4431BDYSi4431BDY

    Q2: FDC645N


    These MOSFETS has RDSon quite low, they are recommended for less than 20Vin

    My DCin is 24V and the maximum voltage applied to the Battery will be 20.5V (avoiding 90-100% duty cycle width pulse)

    I think those mosfets will work (rated to 30V I guess), but for sure there are another mosfets more adequate for this scenario...


    We talk about Vin<20V... and I have 24V


    extract from page 16 datasheet:


    "For VIN < 20V the high current efficiency generally improves with larger MOSFETs, while for VIN > 20V the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CRSS actually provides higher efficiency. The synchronous MOSFET losses are greatest at high input voltage or during a shortcircuit when the duty cycle in this switch in nearly 100%"




    Thanks for the patience

  • 3. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    shabaz Level 15

    Ah, sorry, I'd gone with your original post "datasheet says that if Vout>20V then the RDSon should be little more high" and assumed it matched the datasheet.

    They've got the formulas in the datasheet to calculate the power dissipation, just plug in the CRSS and RDS(ON) values from the MOSFET datasheets (CRSS is in

    a graph) (and they've got an approximation for the temperature dependency too) and an estimate for the constant k that they use in the formulas is there too.


    I ran the formula for the P-ch, and got a value of the order of 50mW, using values of 20.5V, 25V, 0.04ohms, 1.5A, 100pF and 300kHz

    for the parameters involved. I could be wrong, so worth double-checking.

  • 4. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4

    True, I say Vout where it should be Vin>20V


    Anyway, thanks for the help but that demo board is for 4 cells max, and the design differences between charging 4 and 5 cells is greater than the expected. That´s why I´m using LTC4008 (instead of LTC4007 li-ion dedicated)... even the mosfets used should change, and I dont find any reference for a recommendation using 5 cell battery charger. can you believe it? well, it is. And why that demoboard limit the charge to 4 cells? when LTC4008 can charge even until 6 cells!. Something happen when crossing the gap... charging more than 4 cells is odd... I´m just taking precautions in this journey my friend



  • 5. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    shabaz Level 15

    True, very good to take precautions for these types of circuits! I really don't like designing chargers

    to be honest (I've seen what happens when things go wrong to batteries).

    I too wondered, but the eval pdf mentions the limit is due to the capacitor ratings on the board.

    I think it's worth prototyping up with the same MOSFETs, and sticking a scope on the MOSFETs

    to confirm no limit is being exceeded on any spikes, and that the power dissipation is as expected.

    The paper exercise seems to indicate it should work with the same MOSFETs.

    Also, any such circuit should be tested at different temperatures. Good luck, and be careful -

    batteries are exceedingly dangerous. Worth monitoring parameters (current, temperature) of them

    while doing any tests.

  • 6. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4



    Totally agree.


    I had also a very crazy experience but with an assasing CC/CC that killed every mosfet plugged to it during months...(finally was a bug in that family of chips) in this case is even worst because the explosives. I will look for Polimer Lithium Batteries (stronger and less explosives) and also test it inside an iron box or something similar...


    I like the idea of later change the mosfets if I discover better ones, but designing first with these "non-fitted" transistors

    The battery pack I will use will have of course all the protections inside (if not, that would be really dangerous indeed!). The only protection not included is the thermistor network that is easy to implement with the LTC4008. So I hope it would not explode!


    Thanks a lot for the tips.




    PS: I leave the topic open, maybe someone appear and has developed a 5 cell battery charger...



  • 7. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4

    Hi again,


    Looking more deeply in the schematic of dc496b I´ve found many interesting tips which helps me a lot in the design. What I´m wondering now is how Q2 works when DCIN is off.

    It´s known that a mosfet gate should never be with the gate in "open-drain"... so asumming the LTC4008 puts ground level to the P-mosfet Q2 when DCIN is off... then Vds=0 (RDson * I)

    That´s how it´s supposed to work for bring the battery voltage to the system load... but then Q1A is the same, p-mosfet will make Vds=0 when Vgate=0... and that´s not the pourpose of that switch (it should be in the other way) ... Should Q1A be n-mosfet?


    Or maybe... this is all wrong and the current flows through the parasitic diode of Q2 losing 0.5V for reaching system load.. (maybe this has more sense) but then again... what does the gate of Q2 in that moment?




    PS: The charger will be always attached to the battery (inside the case) and only be ON when plugging with a cable for charging...

  • 8. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    shabaz Level 15

    The easiest way of looking at it, is thinking just about VGS. In this case, looking at the circuit, pretend that the source is at the top (I know it isn't, but they are using the trick that the MOSFET is fine in either direction for this part of the discussion, but later they will employ the internal diode). Internally to the IC, the source is connected to the gate by a resistance. In other words, VGS is close to zero volts, and this means that when there is no power, the MOSFET is off. The reason why the MOSFET is actually in the other direction is because you want to block the battery when the input voltage is low. Internally somewhere they control the behavior by pulling the gate negative (along with enabling the PWM to the other MOSFETs) when they wish to enable the charge. That's not what is shown in their block diag, but usually those are a high-level diagram (simplification). Q1A is a P-ch device too, but in a more usual classic configuration. Together with the MOSFET below it, they use PWM to switch the supply to the converter circuitry. The TGATE pin is pulled low internally to switch that Q1A device on.

  • 9. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4

    Quite strange but anyway it has sense

    So, the only reason I see for connecting the Load to the "system Load" -> Vout in the schematic, is when the battery is totally empty and you plug the cable for charge. Battery voltage maybe in that moment is very low due to the limiting function. But the user, in that moment will have all the power he needs for operating taking the power directly from DCIN.

    I was thinking on connecting Vout directly to the Battery -> "connect the Load directly to the battery" but if the user has the battery totally empty he would need to wait for the battery to charge a little for starting to use the equipment and sometimes there is no time to wait...


    So I thought to put a mechanical Relay between the battery and the charger for avoiding losses when all the system is off, and when connecting the cable for charge, the relay should connect the battery to the charger. This is a really bad idea because of the inductor. For sure the relay would kill the mosfets when connecting/disconnecting... If current is flowing and the relay cuts, the inductor would generate high peak voltages... I know that the mosfets does the same. But I fear that the relay is more dangerous than the mosfet itself...


    It seems that when system is off Q1A will prevent current flowing from the battery to the charger, draining all the system only 15uA which is not so bad, so maybe is better to dont put a relay there...

  • 10. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    mgrossman e14 Expert

    Notes on this thread from LTC factory applications. NOt from me.


    There are a number of misinformed thoughts here in this thread. The issues at hand are a power supply book unto themselves. Here are my responses to the most salient points.

    • The DS does not at all relate the ripple with the RDSon. These are fundamentally different quantities. RDSon is the effective steady-state on-state resistance of a MOSFET. Output (voltage) switching ripple in a PWM circuit is a function of the on/off states of the MOSFET pair in conjunction with the inductor. This voltage pulsing causes the current through the inductor to go up and down in a triangular manner. If this were directly applied to the output, you could have horrible results. Thus it is filtered with the output capacitor. What I believe he is referring to is actually the ESR (equivalent series resistance). This is discussed in pg. 14-15. (The value of the capacitance and voltage rating largely affects the sort of expected ESR range given a specific part family due to the physical construction of the capacitor.) Choose the capacitor appropriately in relationship to the “acceptable” amount of the voltage ripple.


    • I am speaking comparatively here and according to what is being addressed by pg. 16. RDSon is indirectly related to is the switching (or transition) loss, which is a function of many parameters, but gets worse with higher VIN because you must use FETs with higher voltage rating. Two things happen:
      • Increasing the voltage rating and trying to keep a similar RDSon inherently increases the gate charge (Qg) and capacitance (CSS, etc.), both contributing to higher losses.
      • The RDSon goes up (usually) if you try to keep the same Qg relative to the previous choice at the lower voltage rating.

    The only the circuit designer can control is to use a FET with lower Qg and Miller capacitance figures, but therein is the trade-off to higher RDSon. For the same voltage rating of FET, you will have to compromise between lowest RDSon and lowest Qg. This is what relates to the efficiency degradation. In the case of the battery charger, we are mostly concerned with finding the happy medium of minimizing the transition losses and maximizing efficiency hit.


    • The FETs used in the demo circuit are acceptable for use in this app, however I would caution the use of an input voltage too close to the FET rating as the switch node will see switching peaks slightly higher depending on the parasitic trace inductance due to layout. It is small and finite, but there are ways to minimize its effects. The switch node can be snubbed out with an RC circuit, but usually the best bet is to minimize the distance from the FETs to the inductor. Or even simpler & safer, use a 40VD-S rated FET.


    • General FET selection tips:
      • Choose your vendor. This is non-trivial if you have bizarre requirements, but in this case the trickiest component is the top FET, which is a P-channel MOSFET. Vishay and Fairchild have been our choices in the past.
      • Choose according to the maximum voltage they will see. In the case of this design, the input and output voltage exceeds 20, so a 30V should be acceptable. However, intuitively you would know that if the input were at 29V, this would be a marginal situation and so should actually be upped to 40V rating. Judgment and experience will show best what is necessary, but a good PCB layout should not see a large excursion. As my personal rule of thumb, I like to estimate at least a ~5V differential between max rating and the max voltage applied to the circuit. As I said, the peak can actually be a little higher due to trace inductance.
      • Next, choose what is an acceptably rated gate drive according to the switching controller in question. Most LTC parts run a 5V gate drive, which is a common rating. This limits the choices further.
      • Choose the preferred package type. In this case, the design could be accomplished with SO-8 or smaller due to the current levels. There should be no reason to “overdesign” the circuit; it can be costly and has diminishing returns for many other reasons.
      • Among the maybe 5-10 choices remaining from the filtering above, choose one with appropriate current rating. In a buck, the worst-case current is seen by the


    • The customer is correct in observing most people don’t care too much of the efficiency of the circuit. Since this is a buck topology, the efficiency can be expected to be in the low- to mid-90s anyway, assuming decent component choices.
    • As for the # of cells, there is a finite number of combinations you want to allow for in the design of a demoboard for common usage. The stacks used back during its design probably did not go past 4-cells, so to design up to 6 may have been a little difficult while still maintaining one inductor value and a simple battery voltage jumper setting. Since the voltage is actually “continuously adjustable” via resistor, only the min/max limits are really of any concern.
    • As observed towards the end of the discussion (as of 2/11) Q1A prevents the load from back-powering the input of the circuit. It is commonly used in our chargers in this manner. The reason it is P-channel so that it does not need an extra “boost” supply to create a bias higher than the rest of the circuit, which will cost extra current to keep on. Q1B allows an alternative path for the battery to power the load rather than going through the top FET (Q2). When you remove input power Q2 will not switch, but it will still conduct battery current through the intrinsic body diode at a true silicon diode drop, causing power loss proportional to the load current, possibly blowing out the top FET due to excessive power dissipation. When the input is removed, R16 pull hard down on the Q1B gate to turn it on. D2 clamps the voltage from gate to source so that the limits are not exceeded on the FET when the input supply returns. It can be an N-channel too, but then you must derive the gate control as a positive bootstrapped voltage on top of the battery voltage to make Vg>Vs, so it is easily accomplished by a P-channel with no extra switching.
    • If you connect the load to the charger output (“charger-fed”), a load cannot exceed battery charge limit. E.g. a 2A charge current limit could never supply more than 2A to a load demanding more current; the output would simply collapse to 0V if the load impedance was low enough. Somewhere in between would still starve the battery for current, causing the charge time to be extended. The arrangement in the demo circuit allows parallel operation of the load while fully charging the battery, assuming the input current limit has not been reached.
    • Relays in battery charger circuits are a bad idea because they take so long to turn on/off, causing the output of the charger to see a high impedance very quickly. This can cause the energy that was trying to make its way to the battery to temporarily raise the output capacitor voltage beyond the programmed amount. This “boosting” can run through the top FET body diode actually getting sent back to the input.
  • 11. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4

    Thank you Mike, it´s a lot of interesting info.  

  • 12. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4


    As most of these mofest are obsolete, here goes my choice for a system with:


    DCIN -> 24V

    V(out)charge max -> 20.5V

    I charge max -> 1.5A (slow charge)

    I load max -> 100-200mA


    Q1A: (Named on Demoboard 496B circuit) SO-8


    SQ4431EY -> Automotive P-Channel 30 V (D-S) 175 °C MOSFET


    Reverse Transfer Capacitance Crss [140 - 175] pF

    Total Gate Chargec Qg [25 - 38] nC


    ABSOLUTE MAXIMUM RATINGS (TC = 25 °C, unless otherwise noted)

    Drain-Source Voltage VDS -30V

    Gate-Source Voltage VGS ±20V

    Continuous Drain Current TC = 25 °C Id -10.8A --- TC = 125 °C -6.2A

    Continuous Source Current (Diode Conduction) IS -5.4A

    Maximum Power Dissipation TC = 25 °C PD 6W --- TC = 125 °C 2W



    Mike, nice trick with R16, Q1B and D2, now it´s clear their pourpose. But as my load will be limited to 200mA (more or less) I thought that Q2 will be happy with the power dissipation, so I´ve removed Q1B.


    Q2: SO-8


    Si4431CDY -> Load Switch/Battery Switch P-Channel 30-V MOSFET


    Reverse Transfer Capacitance Crss 145 pF

    Total Gate Chargec Qg [13 - 38] nC


    ABSOLUTE MAXIMUM RATINGS (TC = 25 °C, unless otherwise noted)

    Drain-Source Voltage VDS -30V

    Gate-Source Voltage VGS ±20V

    Continuous Drain Current Id from -6A to -9A

    Continuous Source Current (Diode Conduction) Is from -2.1A to -3.5A  <---- It should be very comfort for 200mA max (I load)

    Maximum Power Dissipation from 2W to 4W


    I have changed the BGATE mosfet to this one..


    Q7: SO-8

    FDS6612A -> Single N-Channel, Logic-Level, PowerTrench(R) MOSFET


    This N-Channel Logic Level MOSFET is produced using Fairchild Semiconductor’s  advanced PowerTrench process that has been especially tailored to minimize the on-state resistance and yet maintain superior switching performance. These devices are well suited for low voltage and battery powered applications where low in-line power loss and fast switching are required.


    Reverse Transfer Capacitance Crss 55 pF

    Total Gate Chargec Qg [5.4 - 7.6] nC  <---- It´s very nice!


    ABSOLUTE MAXIMUM RATINGS (TC = 25 °C, unless otherwise noted)

    Drain-Source Voltage VDS 30V

    Gate-Source Voltage VGS ±20V

    Continuous Drain Current Id from 8.4A (continuous) to 40A (pulsed)

    Continuous Source Current (Diode Conduction) Is 2.1A

    Maximum Power Dissipation from 1W to 2.5W

    Clearly I have chosen to stay in max Vds 30V. I assume that he ripple in the DCIn of 24V should not be beyond  29-30V.


    As I charge with a low current of 1.5A max, and the batteries are of 4,4A/h, plus the frecuency of charge will be low (two times per month estimated)... those conditions drove me to choose 30Vds max. Because the friendly conditions...if not I would choose 40Vds max.






  • 13. Re: MOSFET recomendation for LTC4008 Li-ion Charger (5 cells pack)
    ndtgenieros Level 4

    These are the results for those mosfets after designing the pcb.


    Vout = 20.4V +- 2mV! (Voltage ripple is quite nice)


    I use 68uH inductor (SRR1260-680M) and two 10uF tantalum Surge-robust capacitors in parallel (T495D106K035ATE300)


    One of the power mosfet´s gate in action:


    0.05useg / division

    2V / div




    I appreciate comments about this waveform, I think it seems pretty nice...