MIC23451 datasheet(15/20 Pages) MICREL

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Micrel, Inc.

MIC23451

November 5, 2013

Revision 1.2

Thermal Considerations

Most applications will not require 2A continuous current

from all outputs at all times, so it is useful to know what

the thermal limits are for various loading profiles.

The allowable overall package dissipation is limited by

the intrinsic thermal resistance of the package (Rθ(J-C))

and the area of copper used to spread heat from the

package case to the ambient surrounding temperature

(Rθ(C-A)). The composite of these two thermal resistances

is Rθ(J-A), which represents the package thermal

resistance with at least 1 square inch of copper ground

plane. From this figure, which for the MIC23451 is

20°C/W, we can calculate maximum internal power

dissipation, as shown in Equation 7:

AMB

JMAX

MAX

Rθ

−

Eq. 7

where:

TJMAX = Maximum junction temp (125°C)

TAMB = Ambient temperature

Rθ(J-A) = 20°C/W

The allowable dissipation tends towards zero as the

ambient temperature increases towards the maximum

operating junction temperature.

The graph of PDMAX vs. ambient temperature could be

drawn quite simply using this equation. However, a more

useful measure is the maximum output current per

regulator vs. ambient temperature. This requires creating

an ‘exchange rate’ between power dissipation per

regulator (PDISS) and its output current (IOUT).

An accurate measure of this function can use the

efficiency curve, as illustrated in Equation 8:

(

)

OUT

LOSS

OUT

−

Eq. 8

where:

= Efficiency

POUT = IOUT.VOUT

To arrive at the internal package dissipation PDISS,

remove the inductor loss PDCR, which is not dissipated

within the package. This does not give a worst case

figure because efficiency is typically measured on a

nominal part at nominal temperatures. The IOUT to PDISS

function used in this case is a synthesized PDISS, which

accounts for worst case values at maximum operating

temperature, as shown in Equation 9.

























−

OUT

DSON_N

OUT

DSON_P

OUT

DISS

Eq. 9

where:

RDSON_P = Maximum RDSON of the high-side, P-Channel

switch at TJMAX

RDSON_N = Maximum RDSON of the low-side, N-Channel

switch at TJMAX

VOUT = Output voltage

VIN = Input voltage

Because ripple current and switching losses are small

with respect to resistive losses at maximum output

current, they can be considered negligible for the purpose

of this method, but could be included if required.

Using the function describing PDISS in terms of IOUT,

substitute PDISS with Equation 7 to form the function of

maximum output current IOUTMAX vs. ambient temperature

TAMB (Equation 10):













−

OUT

DSON_N

OUT

DSON_P

AMB

JMAX

OUTMAX

Rθ

Eq. 10

The curves shown in the “Typical Characteristics” section

are plots of this function adjusted to account for 1, 2, or 3

regulators running simultaneously.

HyperLight Load Mode

Each regulator in the MIC23451 uses a minimum on and

off time proprietary control loop (patented by Micrel).

When the output voltage falls below the regulation

threshold, the error comparator begins a switching cycle

that turns the PMOS on and keeps it on for the duration

of the minimum-on-time. This increases the output

voltage. If the output voltage is over the regulation

threshold, then the error comparator turns the PMOS off

for a minimum-off-time until the output drops below the

threshold. The NMOS acts as an ideal rectifier that

conducts when the PMOS is off. Using an NMOS switch

instead of a diode allows for lower voltage drop across

the switching device when it is on. The asynchronous

switching combination between the PMOS and the

NMOS allows the control loop to work in discontinuous

mode for light load operations. In discontinuous mode,

the MIC23451 works in pulse-frequency modulation

(PFM) to regulate the output. As the output current

increases, the off-time decreases, which provides more

energy to the output. This switching scheme improves the

efficiency of MIC23451 during light load currents by

switching only when it is needed. As the load current

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