SC339 datasheet(8/15 Pages) SEMTECH

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SC339

PRELIMINARY

POWER MANAGEMENT

Table 1 lists recommended resistor values for some stan-

dard output voltages. All resistors are 1%, 1/10W.

The maximum output voltage that can be obtained

from each output is determined by the input supply

voltage and the R

DS(ON) and gate threshold voltage of the

external MOSFET. Assuming that the MOSFET gate

threshold voltage is sufficiently low for the output

voltage chosen and the worst-case drive voltage, V

OUT(MAX)

is given by:

OUT (MAX) = VDRAIN(MIN) ― IOUT(MAX) • RDSON(MAX)

Recommended Resistor Values For SC339

Design Example

Goal: 1.05V±5% @ up to 2.5A from 1.2V±5% and 5V±5%

Solution 1: No Passive Droop

Total window for DC error, ripple and transient is

±52.5mV.

Since this device is linear and assuming that it has been

designed to not ever enter dropout, there is negligible ripple

on the output.

The DC error for this output is the sum of:

ADJ accuracy = ±1% = ±10.5mV

Feedback chain tolerance = ±1% = ±10.5mV

Load regulation = ±0.25% = ±2.6mV

Resistors per Table 1 should be 11.0k

Ω (top) and 10.0kΩ

(bottom).

OUT (V)

R1 or R3 (kΩ)

R2 or R4 (kΩ)

1.05

11.0

10.0

1.2

14.0

10.0

1.5

20.0

10.0

2.5

45.3

11.3

3.3

63.4

11.3

Applications Information (Cont.)

Total DC error = ±2.25% = 23.6mV

This leaves ±2.75% = 28.875mV for the load transient ESR

spike, therefore:

28.875mV

ESR(MAX) =

—————— =

11.55m

2.5A

Bulk capacitance required is given by:

dl •

BULK(MIN) = ——— = μF

Where dI is the maximum load current step, t is the

maximum regulator response time and dV is the

allowable voltage droop. Therefore with dI = 2.5A,

t = 1μs, and dV = 28.875mV:

2.5 • 1 • 10¯

BULK(MIN) = ———————— = 87μF

28.875

• 10¯

So if we use 1% V

OUT set resistors we would select

100μF, 12m

Ω POSCAP for output capacitance (which

assumes that local ceramic bypass capacitors will

absorb the balance of the (12 - 11.55)m

Ω ESR

requirement - otherwise 10m

Ω capacitors should be

used).

If we use 0.1% set resistors, then the total DC error

becomes ±1.35% = ±15.75mV, leaving ±3.65% = 38.33mV

for the ESR spike. In this case:

38.33mV

ESR(MAX) =

———— =

15.33m

Ω and,

2.5A

2.5 • 1 • 10¯

BULK(MIN)

——————— = 65

μF

38.33 • 10¯3

So for 0.1% resistors we could use 1 x 100μF, 15m

POSCAP for output capacitance.

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