ISL6236A Datasheet, PDF(32/37 Page) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

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ISL6236A Datasheet, PDF (32/37 Pages) Intersil Corporation – High-Efficiency, Quad-Output, Main Power Supply Controllers for Notebook Computers

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ISL6236A

Adjustable-Output Feedback (Dual-Mode FB)

Connect FB1 to GND to enable the fixed 5V or tie FB1 to

VCC to set the fixed 1.5V output. Connect a resistive

voltage-divider at FB1 between OUT1 and GND to adjust the

respective output voltage between 0.7V and 5.5V (see

Figure 77). Choose R2 to be approximately 10k and solve for

R1 using Equation 6.

V-----O----U----T----1-

VFB1

1â

(EQ. 6)

where VFB1 = 0.7V nominal.

Likewise, connect REFIN2 to VCC to enable the fixed 3.3V

or tie REFIN2 to VREF3 to set the fixed 1.05V output. Set

REFIN2 from 0V to 2.50V for SMPS2 tracking mode (see

Figure 78).

------V----R--------

VOUT2

1â â

(EQ. 7)

where:

â¢ VR = 2V nominal (if tied to REF)

â¢ VR = 3.3V nominal (if tied to VREF3)

Design Procedure

Establish the input voltage range and maximum load current

before choosing an inductor and its associated ripple current

ratio (LIR). The following four factors dictate the rest of the

design:

1. Input Voltage Range. The maximum value (VIN(MAX))

must accommodate the maximum AC adapter voltage.

The minimum value (VIN(MIN)) must account for the

lowest input voltage after drops due to connectors, fuses

and battery selector switches. Lower input voltages result

in better efficiency.

2. Maximum Load Current. The peak load current

(ILOAD(MAX)) determines the instantaneous component

stress and filtering requirements and thus drives output

capacitor selection, inductor saturation rating and the

design of the current-limit circuit. The continuous load

current (ILOAD) determines the thermal stress and drives

the selection of input capacitors, MOSFETs and other

critical heat-contributing components.

3. Switching Frequency. This choice determines the basic

trade-off between size and efficiency. The optimal

frequency is largely a function of maximum input voltage

and MOSFET switching losses.

4. Inductor Ripple Current Ratio (LIR). LIR is the ratio of

the peak-peak ripple current to the average inductor

current. Size and efficiency trade-offs must be

considered when setting the inductor ripple current ratio.

Low inductor values cause large ripple currents, resulting

in the smallest size, but poor efficiency and high output

noise. Also, total output ripple above 3.5% of the output

regulation will cause controller to trigger out-of-bound

condition. The minimum practical inductor value is one

that causes the circuit to operate at critical conduction

(where the inductor current just touches zero with every

cycle at maximum load). Inductor values lower than this

grant no further size-reduction benefit.

The ISL6236A pulse-skipping algorithm (SKIP = GND)

initiates skip mode at the critical conduction point, so the

inductor's operating point also determines the load

current at which PWM/PFM switchover occurs. The

optimum LIR point is usually found between 25% and

50% ripple current.

VIN

UGATE1

ISL6236A

LGATE1

OUT1

FB1

FIGURE 77. SETTING VOUT1 WITH A RESISTOR DIVIDER

VIN

UGAUTGEA2TE2

ISL88732

ISL68283763A3

ISL88734

LGALTGEA2TE2

VOOUUTOTU2T2

REFRINFE2BFIN2

OUT2

FIGURE 78. SETTING VOUT2 WITH A VOLTAGE DIVIDER FOR

TRACKING

FN6453.3

March 18, 2008

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