Low-Cost, Dual-Output, Step-Down

Controller with Fast Transient Response

18

______________________________________________________________________________________

Power-Up and Sequencing

The MAX8664 features an OSC/EN12 input that is used

both for setting the switching frequency and as an

enable input for both controllers. A resistor from

OSC/EN12 to GND sets the switching frequency, and

when OSC/EN12 is high impedance, both controllers

enter low-power shutdown mode. This is easily

achieved with a transistor between the resistor and

GND. Figure 6a shows the startup configuration with

both controllers use the internal reference.

For tracking applications, connect REFIN2 to the center

of a resistive voltage-divider between the output of con-

troller 1 and GND. See Figure 6b. In this application,

the output of regulator 2 tracks the output voltage of

controller 1. The voltage-divider resistors set the

the VTT supply of DDR memory.

Figure 6c shows one method of sequencing the out-

puts. Output 1 rises first. When PWRGD goes high, the

transistors allow the external reference to drive REFIN2

and output 2 rises. The circuit in Figure 6d functions

similarly, except the enable signal is supplied externally

instead of being driven by the PWRGD signal.

Table 5. Component List for Figure 5

DESIGNATION

QTY

DESCRIPTION

C1, C13

2

0.01µF, 10V X7R ceramic

capacitors

C2, C3, C14, C15

4

10µF, 25V X5R ceramic capacitors

C5

1

1µF, 25V X5R ceramic capacitor

C6

1

1µF, 10V X5R ceramic capacitor

C7, C16

2

0.22µF, 10V X7R ceramic

capacitors

C8, C17

2

4700pF, 10V X7R ceramic

capacitors

C9, C18

2

470µF/2.5V POSCAP capacitors

Sanyo 2R5TPD470M6

C10, C19

2

10µF, 6.3V X5R ceramic capacitors

C11, C20

2

1000pF, 25V X7R ceramic

capacitors

C12

1

4.7µF, 10V X5R ceramic capacitor

DESIGNATION

QTY

DESCRIPTION

L1, L2

2

1.43µH, 4.52m

Ω inductors

Panasonic ETQP3H1E4BFA

N1–N4

4

n-channel MOSFETs

IRF7821 (8-pin SOs)

N5

1

n-channel MOSFET

2N7002 (SOT23)

R1

1

2.87k

Ω ±1% resistor (0402 or 0603)

R2

1

40.2k

Ω ±1% resistor (0402 or 0603)

R3, R11

2

10k

Ω ±1% resistors (0402 or 0603)

R4

1

5.36k

Ω ±1% resistor (0402 or 0603)

R5, R10

2

2.0

Ω ±5% resistors (1206)

R6

1

2.26k

Ω ±1% resistor (0402 or 0603)

R7

1

10k

Ω ±5% resistor (0402 or 0603)

R8

1

75k

Ω ±1% resistor (0402 or 0603)

R9

1

25.5k

Ω ±1% resistor (0402 or 0603)

R12

1

9.53k

Ω ±1% resistor (0402 or 0603)

REFIN2

OSC/EN12

ON

OFF

CHIP

ENABLE

CHIP

ENABLE

PWRGD

MAX8664

Figure 6a. Two Independent Output Startup and Shutdown Waveforms