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MAX863EEE

Dual, High-Efficiency, PFM, Step-Up DC-DC Controller

Maxim Integrated 

Dual, High-Efficiency, PFM, Step-Up

DC-DC Controller

minimum input voltages. Estimate the maximum input

currents for each output based on the minimum input

voltage and desired output power:

VOUT x IOUT

IIN,DC(MAX) ≅ 0.8 x VIN(MIN)

where 0.8 is chosen as a working value for the nominal

efficiency. The power source must be capable of deliv-

ering the sum of the maximum input currents of both

DC-DC converters.

Determine the Peak Switching Current

(Graphical Method)

The peak switching current set by RSENSE determines

the amount of energy transferred from the input on

each cycle. For 3.3V, 5V, 12V, and 24V output circuits,

the peak current can be selected using the output cur-

rent curves shown in Figures 5a–5d.

Determine the Peak Switching Current and

Inductance (Analytical Method)

The following boost-circuit equations are useful when

the desired output voltage differs from those listed in

Figure 5. They allow trading off peak current and induc-

tor value in consideration of component availability,

size, and cost.

Begin by calculating the minimum allowable ratio of

inductor AC ripple current to peak current, ξMIN

(Figure 6):

ξMIN =

tOFF(MIN)

tON(MAX)

x VOUT − VIN(MIN)

VIN(MIN)

where tOFF(MIN) = 2µs and tON(MAX) = 17.5µs.

Select a value for ξ greater than ξMIN. If ξMIN is less

than 1, an acceptable choice is (ξMIN + 1) / 2. If ξMIN is

greater than 1, values between ξMIN and 2 x ξMIN are

acceptable (1.5 x ξMIN, for example). Values greater

than 1 represent designs with full-load operation in dis-

continuous-conduction mode.

Now calculate the peak switching current and induc-

tance. If ξMIN ≤ ξ ≤ 1, use:

( ) IPEAK

= IIN,DC MAX

x

2

2- ξ

For ξ ≥ 1%, use:

( ) ( ) IPEAK

=

2 x IIN,DC MAX

x

VOUT + VIN x

VOUT

ξ −1

ξMIN

=

∆IL

IPEAK

∆IL

IPEAK

t

Figure 6. Ratio of Inductor AC Ripple Current to Peak Current

The suggested inductor value is:

L≅





VOUT

-

VIN(MIN)





x tOFF(MIN)

IPEAK x ξ

Round L up to the next standard inductor value.

Choose RSENSE

The peak switching current is set by RSENSE (R1 and

R2 in Figure 2):

RSENSE ≤

VCS(MIN)

IPEAK

= 85mV

IPEAK

Verify that you’ve selected the correct RSENSE by test-

ing the prototype using the minimum input voltage

while supplying the maximum output current. If the out-

put voltage droops, then decrease the value of the cur-

rent-sense resistor and adjust the other components as

necessary.

The current-sense resistor must be a small, low-induc-

tance type such as a surface-mount metal-strip resistor.

Do not use wire-wound resistors, since their high induc-

tance will corrupt the current feedback signal. In order

to allow use of standard resistor values, round RSENSE

to the next lowest value.

The current-sense resistor’s power rating should be

higher than:

( ) V 2 CS MAX

RPOWER RATING = RSENSE

______________________________________________________________________________________ 11

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