NCP5331 datasheet(34/38 Pages) ONSEMI

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NCP5331

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Equation 28 is used to calculate the heat sink thermal

impedances necessary to maintain less than the specified

maximum junction temperatures at 55

°C ambient.

qCNTRLSA t (120 * 55°C) 1.48 W * 1.65°C W

+ 42.3°C W

qSYNCHSA t (120 * 55°C) 0.85 W * 1.65°C W

+ 74.8°C W per MOSFET

or 37.4

°C W per phase for two MOSFETs phase

If board area permits, a cost effective heatsink could be

formed by using a TO-263 mounting pad of at least 2.0 in2

(1282 mm2) for the upper and lower MOSFETs on a

single-sided, 1 oz copper PCB. The total required pad area

would be slightly less if the area were divided evenly

between top and bottom layers with multiple thermal vias

joining the two areas. To conserve board space, AAVID

offers clip-on heatsinks for TO-220 thru-hole packages.

Examples of these heatsinks include #577002 (1

″× 0.75″ ×

0.25

″, 33°C/W at 2 W) and #591302 (0.75″× 0.5″× 0.5″,

°C/W at 2 W).

6. Adaptive Voltage Positioning

First, to achieve the 200 kHz switching frequency, use

Figure 5 to determine that a 51 k

W resistor is needed for

ROSC. Then, use Figure 6 to find the VFB bias current at the

corresponding value of ROSC. In this example, the 51 kΩ

ROSC resistor results in a VFB bias current of approximately

7.0

µA. Knowing the VFB bias current, one can calculate the

required values for RF1 and RDRP using Equation 29 through

Equation 31.

The no-load position is easily set using Equation 29.

RVFBK + DVNO- LOAD IBIASVFB

+ +25 mV 7.0 mA

+ 3.6 kW

(29)

For inductive current sensing, the designer must calculate

the inductor’s resistance (RL) and approximate any

resistance added by the circuit board (RPCB). We found the

inductor’s nominal resistance in Section 2 (0.965 m

Ω). In

this example, we assume 0.2 m

W for the circuit board

resistance (RPCB). With this information, Equation 30 can

be used to calculate the increase at the VDRP pin at full load.

DVDRP + IO,MAX @ (RL ) RPCB) @ GVDRP

+ 52 A @ (0.965 mW ) 0.2 mW) @ 4.2 V V

+ 0.254 mV

(30)

RDRP can then be calculated from Equation 31.

RDRP +

DVDRP

(IBIASVFB ) DVCORE,FULL- LOAD RF1)

(31)

+ 254 mV (7.0 mA ) 37 mV 3.6 kW)

+ 14.7 kW

7. Current Sensing

Choose the current sense network (RSx, CSx, x = 1 or 2) to

satisfy

RSx @ CSx + Lo (RL ) RPCB)

(32)

Equation 32 will be most accurate for better iron powder

core material (such as the -8 from Micrometals). This

material is very consistent with dc current and frequency.

Less expensive core materials (such as the -52 from

Micrometals) change their characteristics with dc current,

ac flux density, and frequency. This material will yield

acceptable converter performance if the current sense time

constant is set lower (longer) than anticipated. As a rule of

thumb, start with approximately twice the resistance (RSx)

or twice the capacitance (CSx) when using the less expensive

core material.

The component values determined thus far are Lo = 828 nH,

RL = 0.965 mΩ, and RPCB = 0.2 mΩ. We choose a convenient

value for CS1 (0.1 µF) and solve for RSx.

RSn

+ 828 nH (0.965 mW ) 0.2 mW) @ 0.1 mF

+ 7.10 kW

After the circuit is constructed, the values of RSx and/or

CSx should be tuned to provide a “square-wave” at the VDRP

pin with minimal overshoot and fast rise time due to a step

change in load current as shown in Figure 35, Figure 36 and

Figure 37. This testing has shown that for a 3 to 25 A

transient, a value of 10.0 k

W will produce the desired square

wave at VDRP.

8. Error Amplifier Tuning

The error amplifier is tuned by adjusting CA1 to provide

an acceptable full-load transient response as shown in

Figure 38, Figure 39 and Figure 40. After a value for CA1 is

chosen, the peak-to-peak voltage ripple on the COMP pin

is examined under full-load to insure less than 20 mVpp as

shown in Figure 41.

9. Current Limit Setting

The maximum inductor resistance, the maximum PCB

resistance, and the maximum current-sense gain determine

the current limit as shown in Equation 34. The maximum

current,

IOUT,LIMIT, was specified in the design

requirements. The maximum inductor resistance occurs at

full load and the highest ambient temperature. This value

was found in the “Output Inductor Section” (1.28 m

Ω). This

analysis assumes the PCB resistance only increases due to

the change in ambient temperature. Component heating will

also increase the PCB temperature but quantifying this

effect is difficult. Lab testing should be used to “fine tune”

the overcurrent threshold.

RPCB,MAX + 0.2 mW @ {1 ) 0.39% °C

@ (100°C * 25°C)}

+ 0.26 mW

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