MIC2827 Datasheet, PDF (24/27 Pages) Micrel Semiconductor – Triple Output PMIC with HyperLight Load™ DCDC, two LDOs, and I2C Control

Micrel, Inc.
product of the quiescent (operating) current and the
supply voltage is another DC loss. The current required
driving the gates on and off at a constant 4MHz
frequency and the switching transitions make up the
switching losses.
Efficiency VOUT=1.8V
100
VIN=3.6V
90
80
70 _VIN=2.7V
60
50
VIN=4.2V
40
30
20
10
0
1
10
100
1000
LOAD CURRENT (mA)
The Figure above shows an efficiency curve. From no
load to 100mA, efficiency losses are dominated by
quiescent current losses, gate drive and transition
losses. By using the HyperLight Load™ mode the
MIC2827 is able to maintain high efficiency at low output
currents.
Over 100mA, efficiency loss is dominated by MOSFET
R_DSON and inductor losses. Higher input supply voltages
will increase the Gate-to-Source threshold on the
internal MOSFETs, thereby reducing the internal R_DSON.
This improves efficiency by reducing DC losses in the
device. All but the inductor losses are inherent to the
device. In which case, inductor selection becomes
increasingly critical in efficiency calculations. As the
inductors are reduced in size, the DC resistance (DCR)
can become quite significant. The DCR losses can be
calculated as follows:
DCR Loss = I_OUT2 × DCR
From that, the loss in efficiency due to inductor
resistance can be calculated as follows:
Efficiency
Loss
=
⎡
⎢^1−
⎢⎣
⎜⎜⎝⎛
V_OUT × I_OUT
V_OUT × I_OUT + L_P_D
⎟⎟⎠⎞⎥⎥⎦⎤
× 100
Efficiency loss due to DCR is minimal at light loads and
gains significance as the load is increased. Inductor
selection becomes a trade-off between efficiency and
size in this case.
HyperLight Load Mode™
The MIC2827 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
MIC2827
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 a 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 MIC2827 works
in pulse frequency modulation (PFM) to regulate the
output. As the output current increases, the off-time
decreases, thus providing more energy to the output.
This switching scheme improves the efficiency of
MIC2827 during light load currents by only switching
when it is needed. As the load current increases, the
MIC2827 goes into continuous conduction mode (CCM)
and switches at a frequency centered at 4MHz. The
equation to calculate the load when the MIC2827 goes
into continuous conduction mode may be approximated
by the following formula:
I_LOAD
>
⎜⎛
⎝
(V_IN
− V_OUT )× D _⎟⎞
2L × f _⎠
As shown in the previous equation, the load at which
MIC2827 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 MIC2827 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 MIC2827 will transition into PWM mode at a
load of approximately 5mA. Under the same condition,
when the inductance is 1µH, the MIC2827 will transition
into PWM mode at approximately 70mA.
Switching Frequency
vs. Load Current
10
L=4.7µH
1
L=2.2µH
0.1
0.01
1
L=1µH
D_VIN =3.6V
V_OUT = 1.8V
10
100
LO AD CURRENT (mA)
1000
July 2009
24
M9999-072709-A