PSAS/ capstone2009/ design/ aps/ charger
  1. Requirements
  2. Internal Charger
  3. Final IC Chosen:
    1. Model : LTC 4007
  4. ICs Not Chosen:
    1. Model: MAX8731A
    2. Model: LTC1759
    3. Model: BQ24705
    4. Model: ISL88731A
    5. Model: BQ24751A
    6. Model: BQ24730
    7. Model: MAX17005/6/15
    8. Model: MAX1909/MAX8725
  5. Calculations
  6. Parts List
  7. External Charger


* MUST meet standard lithium charge/discharge safety requirements (thermal, voltage, current, time, fuse)
* MUST be able to charge at (,1C,C/2) rates at internal air temperature of 50 deg C.
* MAY equalize cells up to some small bypass power (e.g., .5W)
* MUST indicate charging status (on/off)
* MAY indicate some kind of charge % (blink rate, color, LED bar graph, etc)

Internal Charger

Final IC Chosen:

Model : LTC 4007

ICs Not Chosen:

Model: MAX8731A

Model: LTC1759

Model: BQ24705

Model: ISL88731A

Model: BQ24751A

Model: BQ24730

Model: MAX17005/6/15

Model: MAX1909/MAX8725


Icharge(max)= ((Vref * 3.01kΩ)/Rprog)-.035V)/Rsense)

Assuming Rprog = 26.7 kΩ, based on needs for C/10 comparator Vref = 1.19 V
Rprog and Vref values from datasheet

Charge is 8.5 A

solving for Rsense we get: Rsense=(1.19V * 3.01kΩ)/26.7kΩ)-.035V)/8.5A) = .15Ω

Parts List

LTC4007 4A Charger

U2101 LTC4357

R2100 Low-pass Resistor used to prevent DC overshoot. Value: 10Ω Value based on previous design calculations. Given we are using the same charger chip; keeping the same value seemed reasonable.

R2101 Referred to Rsense in LTC4007 datasheet Value 1 mΩ Using equation of I2R = P, 8.52(.001) = .07225

R2102,2103 Value: 3.01k Based on LTC4007 datasheet values

R2104,2105,2106,2107,2108,2109,2110 Value: 47k Based on need for PIC in previous design

R2111,2112 The two components added together equal Rprog in LTC4007 datasheet Value: 82.5k Using equation for Rprog, got 82.343k total, previois design had 30.1k and 51.1k, which equals 81.2k , which is close so will add .5k to each to get within range, thus 30.6k and 51.6k respectively.

R2113 Value: 6.04k Based on LTC4007 datasheet

R2114 Value: 309k Using equation found in Rt pin description, gives 2.006 hours timer period, 308k better, but not a standard value.

R2115 Value: 5k Based on value in datasheet, used to calculate Rcl

R2116 Value : .168 ohms Based on 8.5 A current, and equation found in Adaptor limiting section

R2117 Value: 11.8k Located before thermistor

R2118 Value: 13.5k Located in parallel with thermistor

R2119 Value 10k

C2100 Bypass capacitor Value: 15 nF Within range given on ltc4007 charger data sheet

C2101 Value:.12uF Based on value given in ltc4007 charger datasheet

C2102 Value: .0047uF Based on values in LTC charger datasheet

C2103,2104 Value: 20uF Based on values in LTC Charger datasheet diagram

C2105 Value .1uF Based on Value in LTC charger datasheet

C2106 Value: 1.2uF Used equations related to thermistor, found in ltc4007 datasheet, calculated at 1.186 uF Digikey number: 399-3119-2-ND

C2107 Value: 39u Battery board C2108 Value: .1u

C2109 100nF

Q2100 N channel mosfet Model #: STS17NH3LL Operates RDS 50 mOhms at 8.5A

Q2101,2102 P channel mosfet Model# : FQB17P06 Operates 120 mOhms at 8.5A

Q2103 N channel mosfet Model#: STS17NH3LL Operates RDS 50 mOhms at 8.5A Previous model only operated at 4.5A


D2100 Model: MA2Q705 Based on previous charger circuit design


L2100 Value 10uH Calculated value less than 10uH not recommended in ltc4007 datasheet, but calculated value was.

External Charger

Near end of project the PSAS decided to look into using an external charger given the capabilities of the charger that they had selected, and space on the board.

Looking at power switches and the internal charger design saw use for switch controller and mosfet as they had the range needed, and by using the enable pin to switch, could control via the microcontroller.

Parts Q2114,2115,2116 See Q2104 for specs Used to control flow based on microcontroller signal 2115,2116 used for loads

C2117,2118,2119 See C2107 for details

U2111,2112,2113 See U2101 for specifics Used to control flow based on microcontroller signal