MIC23156 Datasheet, PDF (15/22 Pages) Micrel Semiconductor – 1.5A, 3MHz Synchronous Buck Regulator with HyperLight Load® and I2C Control for Dynamic Voltage Scaling

Micrel, Inc.
HyperLight Load Mode
The MIC23156 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 synchronous 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 MIC23156 works
in HyperLight Load 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 MIC23156 during light load
currents by only switching when it is needed. As the load
current increases, the MIC23156 goes into continuous
conduction mode (CCM) and switches at a frequency
centered at 3MHz. The equation to calculate the load
when the MIC23156 goes into continuous conduction
mode may be approximated by Equation 5:
I_LOAD
>


(V_IN
− V_OUT
2L × f
)×D


Eq. 5
As shown in Equation 5, the load at which the MIC23156
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). As
shown in Figure 4, as the output current increases, the
switching frequency also increases until the MIC23156
goes from HyperLight Load mode to PWM mode at
approximately 200mA. The MIC23156 will switch at a
relatively constant frequency around 3MHz once the
output current is over 200mA.
MIC23156
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
10
Switching Frequency
vs. Output Current
1.0µH
2.2µH
V_OUT = 1.8V
C_OUT = 2.2µF
100
1000
OUTPUT CURRENT (mA)
10000
Figure 4. SW Frequency vs. Output Current
Output Voltage Setting
The MIC23156 features dynamic voltage scaling and
setting hardware that allow the output voltage of the buck
regulator to be changed on the fly in increments of 10mV.
The output voltage is set according to one of two
registers that behave identically; BUCK_OUT1 when
VSEL = 0 and BUCK_OUT2 when VSEL = 1. If the
BUCK_OUT value is changed while the VSEL is selected
and regulator is enabled, then the output voltage will
immediately change to the new value using dynamic
voltage scaling (DVS). Equation 6 describes the
relationship between the register value and the output
voltage:
V_OUT = 0.7 + (0.01 × REG_{BUCK_OUT})
Eq. 6
Note that the maximum output voltage is 2.4V
corresponding to a register setting of 170 (0b10101010,
0XAA). An example of this calculation is demonstrated in
the Calculating DAC Voltage Code sub-section.
I²C Interface
Figure 5 shows the communications required for write
and read operations via the I²C interface. The black lines
show master communications and the red lines show the
slave communications. During a write operation, the
master must drive SDA and SCL for all stages except the
acknowledgement (A) shown in red, which are provided
by the slave (MIC23156).
The read operation begins first with a data-less write to
select the register address from which to read. A restart
sequence is issued followed by a read command and a
data read.
April 22, 2013
15
Revision 1.0