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ADP3171

Synchronous Buck Controller with Dual Linear Regulator Controllers

Analog Devices 

ADP3171

5V

5VSB

D1

MBR052LT1

12V

D2

MBR052LT1

5VSB

Q1

2N7000

C1

1␮F

C2

10␮F

C3

100pF

+ C5

1000␮F

+ C6

4.7␮F

C7

100nF

C8

22␮F

L1

1.7␮H

1 GND

DRVH 14

2 PWRGD DRVL 13

3 LRFB1

VCC 12

4 LRDRV1 LRFB2 11

5 FB

LRDRV2 10

6 CS–

COMP 9

7 CS+

CT 8

C9

150pF

R1

220⍀

C3

1nF

R2

220⍀

R3 R4

1k⍀ 249⍀

Q2

FDS6982

C10

100pF

R8

10k⍀

R9

8.25k⍀

5VSB

Q3

IRFU014

C12

1␮F

+ C13

220␮F

R6

820␮F ؋ 4

7.5m⍀ 7.5m⍀ ESR (EACH)

C14 C15 C16 C17

1.5V, 5A

3.3VSB, 1.5A

1.5VSB, 35mA

Figure 3. Pentium® III Auxiliary Supply Generating 1.5 V, 1.5 V Standby, and 3.3 V Standby

be disabled in standby mode, but the linear regulators will

begin conducting once VCC rises above about 1 V. During

startup, the linear outputs will track the 3.3 V supply up until

they reach their respective regulation points, regardless of the

state of the 12 V supply. Once the 12 V supply has exceeded

the 5 VSB supply, the controller IC will track the 12 V supply.

Once the 12 V supply has risen above the UVLO value, the

switching regulator will begin its start-up sequence.

tOFF

=



1 –

VOUT

VIN





×

1

fNOM

(1)

tOFF

=



1

–

1.5 V

5V





×

1

200 kHz

= 3.5 µs

The timing capacitor can be calculated from the equation:

APPLICATION INFORMATION

Specifications for a Design Example

The design parameters for a typical auxiliary supply for a Pen-

tium III application (shown in Figure 3) are as follows:

Input Voltage: (VIN) = 5 V

Auxiliary Input: (VCC) = 12 V

Main Output: (VOUT) = 1.5 V @ 5 A

LDO 1 Output: (1.5 VSB) = 1.5 V @ 35 mA

LDO 2 Output: (3.3 VSB) = 3.3 V @ 1.5 A

CT Selection for Operating Frequency

The ADP3171 uses a constant off time architecture, with tOFF

determined by an external timing capacitor CT. Each time the

high side N-channel MOSFET switch turns on, the voltage across

CT is reset to approximately 0 V. During the off time, CT is

charged by a constant current of 150 µA. Once CT reaches 3.0 V,

a new on time cycle is initiated. The value of the off time is

calculated using the continuous-mode operating frequency.

Assuming a nominal operating frequency (fNOM) of 200 kHz at

an output voltage of 1.5 V, the corresponding off time is:

CT

= tOFF × ICT

VT(TH)

= 3.5 µs × 150 µA = 175 pF

3V

(2)

(3) fMIN

=1

tOFF

×

VIN – IO( MAX ) × (RDS (ON )HSF + RSENSE + RL ) – VOUT

VIN – IO( MAX ) × (RDS (ON )HSF + RSENSE + RL – RDS (ON )LSF )

fMIN

=1×

3.5 µs

5V – 5 A × (15 mΩ + 7.5 mΩ + 3 mΩ) – 1.5V = 192 kHz

5V – 5 A × (15 mΩ + 7.5 mΩ + 3 mΩ – 28 mΩ)

The nearest standard value is 150 pF. The converter operates at

the nominal operating frequency only at the above specified VOUT

and at light load. At higher values of VOUT, or under heavy load,

the operating frequency decreases due to the parasitic voltage

drops across the power devices. The actual minimum frequency

at VOUT = 1.5 V is calculated to be 192 kHz (see Equation 3),

where:

RDS(ON)HSF is the resistance of the high side MOSFET

(estimated value: 15 mΩ)

RDS(ON)LSF is the resistance of the low side MOSFET

(estimated value: 28 mΩ)

RSENSE is the resistance of the sense resistor

(estimated value: 7.5 mΩ)

RL is the resistance of the inductor (estimated value: 3 mΩ)

Pentium is a registered trademark of Intel Corporation.

–6–

REV. 0

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