Datasheet

MAX16993 Step-Down Controller with
Dual 2.1MHz Step-Down DC-DC Converters
www.maximintegrated.com
Maxim Integrated │ 16
Skip mode helps improve efficiency in light-load appli-
cations by allowing the converters to turn on the high-
side switch only when the output voltage falls below a
set threshold. As such, the converter does not switch
MOSFETs on and off as often as is the case in the FPWM
mode. Consequently, the gate charge and switching
losses are much lower in skip mode.
Current-Limit/Short-Circuit Protection
Buck converters 2 and 3 feature current limit that protects
the device against short-circuit and overload conditions at
their outputs. The current limit value is dependent on the
version selected, 1.5A or 3.0A maximum DC current. See
the Selector Guide for the current limit value of the chosen
option and the Electrical Characteristics table for the cor-
responding current limit. In the event of a short-circuit or
overload condition at an output, the high-side MOSFET
remains on until the inductor current reaches the high-
side MOSFET’s current-limit threshold. The converter
then turns on the low-side MOSFET and the inductor cur-
rent ramps down.
The converter allows the low-side MOSFET to turn off
only when the inductor current ramps down to the low-
side MOSFET’s current threshold. This cycle repeats until
the short or overload condition is removed.
Applications Information
OUT1 Adjustable Output-Voltage Option
The device’s adjustable output-voltage version (see the
Selector Guide for details) allows the customer to set
OUT1 voltage between 3.0V and 5.5V. Connect a resis-
tive divider from OUT1 to FB1 to GND to set the output
voltage (Figure 3). Select R2 (FB1 to GND resistor) less
than or equal to 100kΩ. Calculate R1 (V
OUT1
to FB1
resistor) with the following equation:
OUT 1
12
FB 1
V
RR 1
V


= −






where V
FB1
= 1.0V (see the Electrical Characteristics).
The external feedback resistive divider must be frequency
compensated for proper operation. Place a capacitor
across R1 in the resistive divider network. Use the follow-
ing equation to determine the value of the capacitor:
if R2/R1 > 1, C1 = C(R2/R1)
else, C1 = C, where C = 10pF.
For fixed output options, connect FB1 to BIAS for the
factory-programmed, fixed output voltage. Connect FB1
to GND for a fixed 3.3V output voltage.
OUT1 Current-Sense Resistor Selection
Choose the current-sense resistor based on the maximum
inductor current ripple (K
INDMAX
) and minimum current-limit
threshold across current-sense resistor (V
LIM1MIN
= 0.1V).
The formula for calculating the current-sense resistor is:
LIM1MIN
MAX
INDMAX
OUTMAX
V
Rcs
K
I (1 )
2
=
×+
where I
OUTMAX
is the maximum load current for Buck 1
and K
INDMAX
is the maximum inductor current ripple.
The maximum inductor current ripple is a function of the
inductor chosen, as well as the operating conditions, and
is typically chosen between 0.3 and 0.4:
[ ] [ ]
SUP OUT
INDMAX
OUTMAX SW 1
(V V ) D
K
I f MHz L µH
−×
=
××
where D is the duty cycle. Below is a numerical exam-
ple to calculate the current-sense resistor in Figure 2.
The maximum inductor current ripple is chosen at the
maximum supply voltage (36V) to be 0.4:
MAX
INDMAX
OUTMAX
0.1
Rcs
K
I1
2
0.1
0.0166
0.4
51
2
=

×+


= = Ω

×+


OUT1 Inductor Selection
Three key inductor parameters must be specified for
operation with the device: inductance value (L), inductor
saturation current (I
SAT
), and DC resistance (R
DCR
). Use
Figure 3. Adjustable OUT1 Voltage Configuration
R1
V
OUT1
C1
OUT1
MAX16993
FB1
R2