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CS5127GDWR16

Dual Output Nonsynchronous Buck Controller with Sync Function and Second Channel Enable

Cherry semiconductor 

Applications Information

Selection of Feedback Lead Divider Resistor Values

The feedback (VFB) leads are connected to external resistor

dividers to set the output voltage. The on-chip error ampli-

fier is referenced to 1.275V, and the resistor divider values

are determined by selecting the desired output voltage

and the value of the divider resistor connected between

the VFB lead and ground.

Resistor R1 is chosen first based on a design trade-off of

system efficiency vs. output voltage accuracy. Low values

of divider resistance consume more current which decreas-

es system efficiency. However, the VFB lead has a 1µA

maximum bias current which can introduce errors in the

output voltage if large resistance values are used. The

approximate value of current sinking through the resistor

divider is given by

1.275V

IV(FB) = R1

The output voltage error that can be expected due to the

bias current is given by

Error Percentage =

(1E - 6) ´ R1

1.275

´ 100%

where R1 is given in ohms. For example, setting R1 = 5K

yields an output voltage error of 0.39% while setting the

feedback divider current at 255µA. Larger currents will

result in reduced error.

Output

Driver

COMP

1.275V

+

R2

-

VFB

R1

GATE

VOUT

variables the designer must consider. Inductance values

between 1µH and 50µH are suitable for use with the CS5127.

Low values within this range minimize the component size

and improve transient response, but larger values reduce

ripple current. Choosing the inductor value requires the

designer to make some choices early in the design. Output

current, output voltage and the input voltage range should

be known in order to make a good choice.

The input voltage range is bracketed by the maximum and

minimum expected values of VIN. Most computer applica-

tions use a fairly well-regulated supply with a typical

output voltage tolerance on the order of ±5%. The values

of VIN(MAX) and VIN(MIN) are used to calculate peak current

and minimum inductance value, respectively. However, if

the supply is well-regulated, these calculations may both

be made using the typical input voltage value with very

little error.

Current in the inductor while operating in the continuous

current mode (CCM) is defined as the load current plus

the inductor ripple current:

IL = IOUT + IRIPPLE

The ripple current waveform is triangular, and the current

is a function of the voltage across the inductor, the switch

on-time and the inductor value. Switch on-time is the duty

cycle divided by the operating frequency, and duty cycle

can be defined as the ratio of VOUT to VIN, such that

IRIPPLE =

(VIN - VOUT)VOUT

f ´ L ´ VIN

The peak current can be described as the load current plus

half of the ripple current. Peak current must be less than

the maximum rated switch current. This limits the maxi-

mum load current that can be provided. It is also

important that the inductor can deliver the peak current

without saturating.

IOUT(MAX) = ISWITCH(MAX) -

(VIN(MAX) - VOUT)VOUT

2f ´ L ´ VIN(MAX)

Figure 3: Feedback resistor divider.

R2 can be sized according to the following formula once

the desired output voltage and the value of R1 have been

determined:

( ) R2 = R1

VOUT

1.275

-1

Selecting the Inductor

There are many factors to consider when choosing the

inductor. Maximum load current, core losses, winding

losses, output voltage ripple, short circuit current, satura-

tion, component height, EMI/EMC and cost are all

Since the peak inductor current must be less than or equal

to the peak switch current, the minimum value of induc-

tance can be calculated:

LMIN =

(VIN(MIN) - VOUT)VOUT

f ´ VIN(MIN) ´ ISWITCH(MAX)

Load Current Transient Response

The theoretical limit on load current transient response is a

function of the inductor value, the load transient and the

voltage across the inductor. In conventionally-controlled

regulators, the actual limit is the time required by the con-

trol loop. Conventional current-mode and voltage-mode

control loops adjust the switch duty cycle over many oscil-

lator periods, often requiring tens or even hundreds of

7

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