MIC23254 Datasheet, PDF (13/17 Pages) Micrel Semiconductor – 4MHz Dual 400mA Synchronous Buck Regulator with Low Input Voltage and HyperLight Load™

Micrel, Inc.
MIC23254
As shown in the previous equation, the load at which
MIC23254 transitions from HyperLight Load™ mode to
PWM mode is a function of the input voltage (V_IN), output
voltage (V_OUT), duty cycle (D), inductance (L) and
frequency (f). This is illustrated in the graph below. Since
the inductance range of MIC23254 is from 0.47µH to
4.7µH, the device may then be tailored to enter HyperLight
Load™ mode or PWM mode at a specific load current by
selecting the appropriate inductance. For example, in the
graph below, when the inductance is 4.7µH the MIC23254
will transition into PWM mode at a load of approximately
5mA. Under the same condition, when the inductance is
1µH, the MIC23254 will transition into PWM mode at
approximately 70mA.
10
4MHz
Switching Frequency
vs. Output Current
L = 4.7µH
1
L = 1µH
Thermal circuits can be considered using these same rules
and can be drawn similarly replacing current sources with
Power dissipation (in Watts), Resistance with Thermal
Resistance (in ^ºC/W) and Voltage sources with
temperature (in ^ºC):
Now replacing the variables in the equation for Vx, we can
find the junction temperature (T_J) from power dissipation,
ambient temperature and the known thermal resistance of
the PCB (Rθ_CA) and the package (Rθ_JC):
( ) T_J = P_DISS ⋅ Rθ_JC + Rθ_CA + T_AMB
0.1
0.01
1
L = 2.2µH
VIN = 3.6V
VOUT = 1.8V
COUT = 4.7µF
10
100
1000
OUTPUT CURRENT (mA)
Power Dissipation Considerations
As with all power devices, the ultimate current rating of the
output is limited by the thermal properties of the package
and the PCB it is mounted on. There is a simple, ohms law
type relationship between thermal resistance, power
dissipation and temperature which are analogous to an
electrical circuit:
As can be seen in the diagram, total thermal resistance
Rθ_JA = Rθ_JC + Rθ_CA. Hence this can also be written:
( ) T_J = P_DISS ⋅ Rθ _JA + T_AMB
P_DISS can be calculated thus:
P_DISS
= P_OUT
⋅(¹
η
− 1)
Where η = Efficiency taken from efficiency curves
Rθ_JC and Rθ_JA are found in the operating ratings section of
the datasheet.
From this simple circuit we can calculate Vx if we know
Isource, Vz and the resistor values, Rxy and Ryz using the
equation:
Vx = Isource ⋅ (Rxy + Ryz) + Vz
May 2010
13
M9999-052510