CS5211GDR14G データシートの表示（PDF） - ON Semiconductor

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CS5211GDR14G

Low Voltage Synchronous Buck Controller

ON Semiconductor 

CS5211

qSA is the sink−to−ambient thermal impedance of the

heatsink assuming direct mounting of the MOSFET (no

thermal “pad” is used).

TJ is the specified maximum allowed junction

temperature.

TA is the worst case ambient operating temperature.

For TO−220 and TO−263 packages, standard FR−4

copper clad circuit boards will have approximate thermal

resistances (qSA) as shown below:

Pad Size

(in2/mm2)

0.5/323

0.75/484

1.0/645

1.5/968

2.0/1290

2.5/1612

Single−Sided

1 oz. Copper

60−65°C/W

55−60°C/W

50−55°C/W

45−50°C/W

38−42°C/W

33−37°C/W

As with any power design, proper laboratory testing

should be performed to insure the design will dissipate the

required power under worst case operating conditions.

Variables considered during testing should include

maximum ambient temperature, minimum airflow,

maximum input voltage, maximum loading, and component

variations (i.e. worst case MOSFET RDS(on)). Also, the

inductors and capacitors share the MOSFET’s heatsinks and

will add heat and raise the temperature of the circuit board

and MOSFET. For any new design, its advisable to have as

much heatsink area as possible − all too often new designs

are found to be too hot and require re−design to add

heatsinking.

Compensation Capacitor Selection

The nominal output current capability of the error amp is

30 mA. This current charging the capacitor on the COMP pin

is used as soft−start for the converter. The COMP pin is going

to ramp up to a voltage level that is within 70 mV of what VFFB

is going to be when in regulation. This is the voltage that will

determine the soft−start. Therefore, the COMP capacitor can

be established by the following relationship:

C + 30 mA

soft start

VFFB(REG)

where:

soft−start = output ramp−up time

VFFB(REG) = VFFB voltage when in regulation

30 mA = COMP output current, typ.

The COMP output current range is given in the data sheet

and will affect the ramp−up time. The value of the capacitor on

the COMP pin will have an effect on the loop response and the

transient response of the converter. Transient response can be

enhanced by the addition of a parallel combination of a resistor

and capacitor between the COMP pin and the comp capacitor.

ROSC Selection

The switching frequency is programmed by selecting the

resistor connected between the ROSC pin and SGND (pin 7).

The grounded side of this resistor should be directly

connected to the SGND pin, without any other currents

flowing between the bottom of the resistor and the pin. Also,

avoid running any noisy signals under the resistor, since

injected noise could cause frequency jitter. The graph in

Figure 6 shows the required resistance to program the

frequency. Below 500 kHz, the following formula is

accurate:

R + 17544ńfSW * 4 kW

where fSW is the switching frequency in kHz.

140

120

100

80

60

40

20

0

0 100 200 300 400 500 600 700 800

Frequency (kHz)

Figure 6. Frequency vs. ROSC

Differential Remote Sense Operation

The ability to implement fully differential remote sense is

provided by the CS5211. The positive remote sense is

implemented by bringing the output remote sense connection

to the positive load connection. A low value resistor is

connected from Vout to the feedback point at the regulator to

provide feedback in the instance when the remote sense point

is not connected.

The negative remote sense connection is provided by

connecting the SGND of the CS5211 to the negative of the load

return. Again, a low value resistor should be connected

between SGND and LGND at the regulator to provide

feedback in the instance when the remote sense point is not

connected. The maximum voltage differential between the

three grounds for this part is 200 mV.

Feedback Divider Selection

The feedback voltage measured at VFB during normal

regulation will be 1.0 V. This voltage is compared to an internal

1.0 V reference and is used to regulate the output voltage. The

bias current into the error amplifier is 1.0 mA max, so select the

resistor values so that this current does not add an excessive

offset voltage.

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