RT8205CGQW datasheet(24/28 Pages) RICHTEK

24 / 28 page

RT8205A/B/C

DS8205A/B/C-06 July 2012

www.richtek.com

is a registered trademark of Richtek Technology Corporation.

P-P

LOAD(MAX)

ESR

≤

VREG5 connects to VOUT1 through an internal switch

only when VOUT1 is above the VREG5 automatic switch

threshold (4.66V). VREG3 connects to VOUT2 through

an internal switch only when VOUT2 is above the VREG3

automatic switch threshold (3.06V). This is the most

effective way when the fixed output voltages are used.

Once VREGx is supplied from VOUTx, the internal linear

regulator turns off. This reduces internal power dissipation

and improves efficiency when the VREGx is powered with

a high input voltage.

Figure 6. Setting VOUTx with a Resistor-Divider

Output Inductor Selection

The switching frequency (on-time) and operating point (%

ripple or LIR) determine the inductor value as shown as

follows :

OUTx

LOAD(MAX)

T(V

)

L =

×−

where LIR is the ratio of the peak to peak ripple current to

the average inductor current.

SAG

OUTx

LOAD

OFF(MIN)

OUTx

OUT

OUTx

OFF(MIN)

)

⎛⎞

Δ× ×

⎜⎟

⎝⎠

⎡⎤

−

⎛⎞

××

−

⎜⎟

⎢⎥

⎝⎠

⎣⎦

Where minimum off-time (TOFF(MIN)) = 300ns (typ.) and K

is from Table 1.

Output Capacitor Selection

The output filter capacitor must have low enough ESR to

meet output ripple and load-transient requirements, yet

have high enough ESR to satisfy stability requirements.

Also, the capacitance value must be high enough to

absorb the inductor energy going from a full-load to no-

load condition without tripping the OVP circuit.

For CPU core voltage converters and other applications

where the output is subject to violent load transients, the

output capacitor's size depends on how much ESR is

needed to prevent the output from dipping too low under a

load transient. Ignoring the sag due to finite capacitance :

PHASEx

LGATEx

VOUTx

VIN

UGATEx

VOUTx

FBx

GND

PGND

Find a low-loss inductor having the lowest possible DC

resistance that fits in the allotted dimensions. Ferrite cores

are often the best choice, although powdered iron is

inexpensive and can work well at 200kHz. The core must

be large enough not to saturate at the peak inductor current

(IPEAK) :

IPEAK = ILOAD(MAX) + [(LIR / 2) x ILOAD(MAX)]

This inductor ripple current also impacts transient-response

performance, especially at low VIN

− VOUTx differences.

Low inductor values allow the inductor current to slew

faster, replenishing charge removed from the output filter

capacitors by a sudden load step. The peak amplitude of

the output transient VSAG is also a function of the output

transient. The VSAG also features a function of the

maximum duty factor, which can be calculated from the

on-time and minimum off-time :

Output Voltage Setting (FBx)

Connect FBx directly to GND or VREG5 to enable the

fixed, SMPS output voltages (3.3V and 5V). Connect a

resistor voltage-divider at the FBx between the VOUTx

and GND to adjust the respective output voltage between

2V and 5.5V (Figure 6). Choose R2 to be approximately

10k

Ω, and solve for R1 using the equation :

OUTx

FBx

= V

⎡⎤

⎛⎞

×+ ⎜⎟

⎢⎥

⎝⎠

⎣⎦

where VFBx is 2V (typ.).

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