SC1406GCTSTR datasheet(15/28 Pages) SEMTECH

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ã 2000 Semtech Corp.

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POWER MANAGEMENT

SC1406G

Requirements:

The critical processor requirements are:

CC650MAXDC = 1.65V

CC650MINDC = 1.485V

CC650MAXTRANS = 1.715V

CC650MINTRANS = 1.485V

CC500MAXDC = 1.45V

CC500MINDC = 1.25V

CC500MAXTRANS = 1.45V

CC500MINTRANS = 1.25V

CC650MAX = 13.6A

10. I

CC650SG = 2.2A

11. I

CC500MAX = 9.5A

12. I

CC500SG = 1.7A

13. dI

CC/dt = 1400A/mS (at the processor. Local decoupling

reduces the requirement at the regulator.)

The system requirements are to minimize the number of

capacitors, and:

ADAPTERMAX = 21V

BATTMIN = 10V

Basic Calculations:

The 0.85% DAC output voltage accuracy accounts for an

uncertainty of 14mV at the 1.60V setting. In addition, 20mV of

resistive drop is expected in the power distribution from the

current sense resistor to the processor. A footnote in the

processor specification reads that the long-term voltage should

never exceed 1.65V. As a result, the nominal value of the no-

load voltage [V (0)] should be set accordingly.

The full-load voltage (V (fl)) is set similarly, including tolerance

and DC drop.

The regulator is to be designed with 40mV of output ripple, so

the effective IMVP voltage drop (V

IMVP) is:

Output Inductor and Capacitor Selection:

Output capacitance and ESR values are a function of transient

requirements and output inductor value. The following figure

illustrates the response of a hysteretic converter to a positive

transient:

Figure 3 - Hysteretic Converter Response to a Positive

Transient

In a hysteretic converter with adaptive voltage positioning, like

the SC1406, two conditions determine if you meet the positive

transient requirements:

The first condition is easy to see if the ESR is too high, the

transient response will fail.

In the second condition, because the hysteretic converter

responds in < 100ns, the capacitor does not droop very far

before the inductor current starts ramping up. (This is not true

of control schemes where time constants in the error amplifier

cause delays.) Once the inductor current starts to rise, the

increasing DV of the capacitor is offset by reduced DV from the

ESR, so DV is constant. If the DV due to the charge taken from

the capacitor before the inductor current reaches the load

current (see the shaded area above) is less than V

IMVP, then the

transient response will pass.

The maximum ESR requirement is:

For the second condition, we need to know the inductor value,

which is a function of the highest desired switching frequency.

The maximum frequency occurs at the highest input voltage. As

a reasonable compromise between efficiency and component

size, a maximum switching frequency of 300kHz is desired.

VNL

VCC650MAXDC

VTOL

−

VNL

1.636 V

VFL

VCC650MINDC

VTOL

VDIST

VFL

1.519 V

VIMVP

VNL

VFL

−

VRIPPLE

−

VIMVP 0.098 V

(

)

VIMVP

ICC650MAX ICC650SG

−

() ESR

⋅

≥

VIMVP deltaV COUT

()

≥

ESRMAX

VIMVP

ICC650MAX ICC650SG

−

()

ESRMAX 8.579 10

−

×Ω

SC1406GCTSTR Datasheet(PDF) 15 Page - Semtech Corporation

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