MIC24046 Datasheet, PDF (18/27 Pages) Micrel Semiconductor – Pin-Programmable, 4.5V − 19V, 5A Step-Down Converter

Micrel, Inc.
Setting the Output Voltage
The MIC24046 output voltage can be programmed by
setting pins VOSET0 and VOSET1, as shown in Table 2.
Table 2. Output Voltage Settings
VOSET1
VOSET0
Output Voltage
0 (GND)
0 (GND)
3.3V
0 (GND)
1 (VDDA)
2.5V (2.49V)
1 (VDDA)
0 (GND)
1.8V
1 (VDDA)
1 (VDDA)
1.5V
0 (GND)
Hi-Z (Open)
1.2V
Hi-Z (Open) 0 (GND)
1.0V
1 (VDDA)
Hi-Z (Open)
0.9V
Hi-Z (Open) 1 (VDDA)
0.8V
Hi-Z (Open) Hi-Z (Open)
0.7V
To achieve accurate output voltage regulation, the
OUTSNS pin (internal feedback divider top terminal)
should be Kelvin-connected as close as possible to the
point-of-regulation top terminal. Since both the internal
reference and the internal feedback divider’s bottom
terminal refer to AGND, it is important to minimize voltage
drops between the AGND and the point-of-regulation
return terminal.
Setting the Current Limit
The MIC24046 valley-mode current limit on the low-side
MOSFET can be programmed by means of ILIM as
shown in Table 3.
Table 3. Current-Limit Setting
ILIM
Low-Side Valley
Current Limit
(Typical Value)
0 (GND)
4.6 A
1 (VDDA)
6.2 A
Hi-Z (Open)
6.8 A
Rated Output
Current
3A
4A
5A
Note that the programmed current-limit values act as
pulse-by-pulse current-limit thresholds on the valley
inductor current. If the inductor current has not decayed
below the threshold at the time the PWM requires a new
ON time, the high-side MOSFET turn-on is either delayed
until the valley current recovers below the threshold or
skipped. Each time the high-side MOSFET turn-on is
skipped, a 4-bit up-down counter is incremented. When
the counter reaches the configuration 1111, a hiccup
sequence is invoked in order to reduce power dissipation
under prolonged short-circuit conditions.
The highest current-limit setting (6.8A) is intended to
comfortably accommodate a 5A application.
MIC24046
Ensure the value of the operating junction temperature
does not exceed the maximum rating in high output
power applications.
Inductor Selection and Slope Compensation
When selecting an inductor, it is important to consider the
following factors:
• Inductance
• Rated current value
• Size requirements
• DC resistance (DCR)
• Core losses
The inductance value is critical to the operation of
MIC24046. Since the MIC24046 is a valley current-mode
regulator, it needs a slope compensation for the stable
current loop operation where duty cycles are below 50%.
Slope compensation is internally programmed according
to the frequency and output voltage selection, assuming
there is a minimum inductance value for the given
operating condition. Table 4 lists the assumed minimum
inductor values recommended for stable current loop
operation. Note that the minimum suggested inductance
values should be met when taking into account inductor
tolerance and its change with current level.
Table 4. Recommended Inductance Values at VIN = 12V
VOUT
SELECTION
FREQUENCY
MINIMUM
INDUCTANCE
400kHz
0.97
0.7V, 0.8V, 0.9V,
565kHz
0.68
1.0V, 1.2V
790kHz
0.49
400kHz
1.51
1.5V, 1.8V
565kHz
1.06
790kHz
0.76
400kHz
2.42
2.49V, 3.3V
565kHz
1.70
790kHz
1.21
The slope compensation is also internally adapted to the
input-output voltage differential.
In practical implementations of valley-current-mode
control, slope compensation is also added to any duty
cycle larger than 50% as part of improving current loop
stability and noise immunity for all input and output
voltage ranges. Consequently, the MIC24046 adds
internal slope compensation signal up to 60% duty cycle.
Above this, no slope compensation is added. For this
reason, the PWM modulator gain exhibits an abrupt
change when the duty cycle exceeds 60%, possibly
leading to some increase in jitter and noise susceptibility.
October 14, 2015
18
Revision 1.1