Rainbow Electronics MAX781 - Manual (Page 17)

R7 can be as large as 0.082
Ω
x 1.01 = 0.0828
Ω
, yield-
ing a minimum peak current limit of 80mV / 0.0828
Ω
=
0.966A.
Use the spreadsheet in Listings 1 and 2 to calculate the
power available at 3OUT as a function of the current-
sense resistor choice.
Listing 1. Spreadsheet for Calculating 3OUT
Current Capability
Parameter Min Max U n i t s
(Cell A1) (Cell B1) (Cell C1)
Current-Limit Threshold 80 120 mV
Current Sense R7 81 83 m Ω
Current Limit 0.964 1.481 A
Switching Frequency 270 340 kHz
Switching Period 2.941 3.704 µs
T1 Primary Inductance 16 24 µH
3OUT Regulation Point 3.170 3.430 V
BATT Input Range 5.000 17.500 V
(Cell A10)
Current Limit 0.964 A
Switching Period 3.704 µs
T1 Primary Inductance 16 µH
3OUT 3.430
BATT 17.500
VHI Load Current 60 mA
T1 Turns Ratio 3.5
T1 Coupling Loss 80 %
(Cell A20)
T1 Ripple Current 0.638 A
T1 Continuous Current 0.325 A
3OUT Current, No VHI load 0.645 A
VHI Load Power 0.926 W
3OUT Guaranteed Current 0.307 A
(Cell A26) (Cell B26) (Cell C26)
Listing 2. Calculating 3OUT Current-Capability
Formulas
B4: +B2/C3
C4: +C2/B3
B6: 1/C5*1000
C6: 1/B5*1000
B12: +B4
C13: +C6
B14: +B7
C15: +C8
C16: +C9
C21: @MIN(+C15/C16*C13*(C16-C15)/B14,B12)
C22: +B12-C21
C23: @IF(C22=0,0.5*(B14/1000000)*C21^2*1000000/
C13/C15,C22+C21/2)
C24: +C15*(C18+1)*C17/1000
C25: +C23-C24/(C19/100)/C15
R7 must have as little series inductance as possible
and be as physically small as possible. 3OUT and CS3
need to Kelvin sense R7. A pair of traces running in
parallel should leave 3OUT and CS3 and diverge only
when they meet R7. Minimize the distance between
R7 and the positive terminal of C6.
Power MOSFETs
M1 and M2 must be logic level, low r DS(ON) , N-channel
power MOSFETs. M1’s drain should be as close as
possible to C15’s positive terminal and M2’s source
should be as close as possible to C15’s ground con-
nection point.
Transformer T1
T1’s primary inductance must be between 10µH and
100µH. The peak current allowed through the primary
with the secondary open circuited must be greater than
the worst-case peak current set by R7. T1’s turns ratio
(number of turns on secondary / number of turns on pri-
mary) should be 3.5. If VHI rises up to 20V when 3OUT
is loaded in operate mode, T1 may have too much
interwinding capacitance. Minimize interwinding
capacitance to prevent energy waste in the VHI clamp
(which clamps VHI to 19V to protect the MAX781).
__________Applications Information
Design Example
Table 7 shows the targets for a typical design require-
ment. Since both PCMCIA slots will not be pro-
grammed at the same time, VPPA and VPPB will never
be at +12V at the same time; thus the worst case for
power consumption is when both 3.3V and VPPA or
VPPB is fully loaded.
Total power consumption = (max 3OUT voltage) x (max
3OUT load current) + (VHI voltage) x (max VPP or load
current) / (transformer efficiency).
MAX781
PDA/Hand-Held Computer Power Controller
______________________________________________________________________________________ 17