CS5305 datasheet(21/33 Pages) ONSEMI

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CS5305

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connection point for a compensation capacitor for the share

adjust amplifier.

CHOOSING EXTERNAL COMPONENTS

FOR THE CS5305

ROCSET and ROSC

The ROSC lead of the CS5305 provides a fixed 1 V

reference to the user. A resistive divider is connected from

ROSC to ground as shown in Figure 35. The center tap of the

divider is connected to the OCSET lead. The total

resistance from the ROSC lead to ground programs the

oscillator frequency for the converter according to the chart

in Figure 36.

The resistive divider also sets a voltage on the OCSET lead.

This voltage programs the module overcurrent trip point. The

module overcurrent comparator, or OC Comparator, uses the

OCSET lead voltage as the reference against which the

module output current signal is compared. The output current

of each phase is given as (VCSx − VCSREF) divided by the

equivalent series resistance of the inductor. The voltage

information (VCSx − VCSREF) is gained up by a factor of 3.7

and summed for all three phases at the non−inverting input of

the OC Comparator. The fault latch is set if the module

overcurrent limit is exceeded. This results in “hiccup−mode”

operation until the overcurrent condition is cleared.

ROCSET

ROSC

OCSET

ROSC = R1 + ROCSET

Figure 35.

Figure 36. FOSC vs. R1 + ROCSET

100

R1 + ROSCSET (kΩ)

300

400

500

600

700

800

900

200

Additionally, the total value of resistance between ROSC

and ground also programs the VFB pin bias current. VFB bias

current is equal to 0.333 V divided by the total resistance

from ROSC to ground. This current is used to generate the

droop function in the adaptive voltage positioning circuitry

and is discussed further in that section.

Current Sense Components

Current sense components are chosen for two reasons.

First, the value of RCSx and CCSx should be chosen to meet

the criterion:

(RCSx)(CCSx) w (L) (ESRL)

where L is the inductor value and ESRL is the inductor

equivalent series resistance. Meeting this criterion will

ensure that the module overcurrent limit is not exceeded

during current transients. Second, RCSx and CCSx should be

chosen to add a small amount of ramp to the system. This

will provide stable, jitter−free operation. The amount of

ramp voltage required depends on several factors: supply

voltage, output voltage (DAC code), switching frequency

and board layout all affect the amount of artificial ramp

required to some degree. The power supply designer should

be aware that choosing the value of artificial ramp is a

trade−off. As artificial ramp amplitude increases, the system

becomes less prone to duty cycle jitter, but transient

response will suffer. Adding 20 mV of artificial ramp is a

good compromise and can be used to start design.

The current sense ramp is generated from the square wave

obtained at the switching node of each phase by using an RC

filter. The RC filter components for the CSx leads should be

chosen to satisfy the following formula:

RCSx CCSx v

(VOUT) 1 * VOUT VCC

(fOSC)(VRAMP)

Choose a convenient standard value for CCSx and solve for

the value of RCSx. Each of the three output phases requires

its own RC combination.

An RC filter is also required for the CSREF connection.

This filter may use the same value of capacitance identified

for the CS1, CS2 and CS3 leads, but the value of resistance

should be one third that of RCSx:

RCSREF + RCSx 3

This change is necessary to compensate for the difference

in bias current between the CSREF lead and each CSx lead.

The schematic in Figure 37 shows the connection of these

components.

RCSx

CCSx

CCSREF

RCSREF = RCSx/3

CSREF

VOUT

Switch

Node x

CSx

Figure 37.

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