Datasheet

MAX16993 Step-Down Controller with
Dual 2.1MHz Step-Down DC-DC Converters
www.maximintegrated.com
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Current-Sense Measurement
For the best current-sense accuracy and overcurrent pro-
tection, use a 1% tolerance current-sense resistor between
the inductor and output, as shown in Figure 2. This con-
figuration constantly monitors the inductor current, allow-
ing accurate current-limit protection. Use low-inductance
current-sense resistors for accurate measurement.
High-Side Gate-Drive Supply (BST1)
The high-side MOSFET is turned on by closing an inter-
nal switch between BST1 and DH1 and transferring the
bootstrap capacitor’s (at BST1) charge to the gate of the
high-side MOSFET. This charge refreshes when the high-
side MOSFET turns off and the LX1 voltage drops down
to ground potential, taking the negative terminal of the
capacitor to the same potential. At this time, the bootstrap
diode recharges the positive terminal of the bootstrap
capacitor. The selected n-channel high-side MOSFET
determines the appropriate boost capacitance values
(C
BST1
in the Typical Operating Circuit) according to the
following equation:
G
BST 1
BST 1
Q
C
V
=
∆
where Q
G
is the total gate charge of the high-side
MOSFET and ΔV
BST1
is the voltage variation allowed
on the high-side MOSFET driver after turn-on. Choose
ΔV
BST1
such that the available gate-drive voltage is not
significantlydegraded(e.g.,ΔV
BST1
= 100mV to 300mV)
when determining C
BST1
. Use a Schottky diode when
efficiency is most important, as this maximizes the gate-
drive voltage. If the quiescent current at high temperature
is important, it may be necessary to use a low-leakage
switching diode.
The boost capacitor should be a low-ESR ceramic capaci-
tor. A minimum value of 100nF works in most cases. A
minimum value of 470nF is recommended when using a
Schottky diode.
Dropout
When OUT1 input voltage is lower than the desired output
voltage, the converter is in dropout mode. Buck 1 continu-
ously draws current from the bootstrap capacitor when the
high-side switch is on. Therefore, the bootstrap capacitor
needs to be refreshed periodically. When in dropout, the
Buck 1 high-side gate drive shuts off every 8µs, at which
point the low-side gate drive turns on for 120ns.
Buck 2 and Buck 3 (OUT2 and OUT3)
Buck converters 2 and 3 are high-efficiency, low-voltage
converters with integrated FETs. They use a PWM
current-mode control scheme that is operated at 2.1MHz
to optimize component size and efficiency, while elimi-
nating AM band intererence. The buck converters can
be configured to deliver 1.5A or 3.0A per channel. They
operate directly from OUT1 and have either fixed or
resistor-programmable (see the Selector Guide) output
voltages that range from 0.8V to 3.95V. Buck 2 and Buck 3
feature low on-resistance internal FETs that contribute to
high efficiency and smaller system cost and board space.
Integration of the p-channel high-side FET enables both
channels to operate with 100% duty cycle when the input
voltage falls to near the output voltage. They feature a
programmable active timeout period (see the Selector
Guide) that adds a fixed delay before the corresponding
RESET_ can go high.
FPWM/Skip Modes
The MAX16993 features an input (SYNC) that puts the
converter either in skip mode or forced PWM (FPWM)
mode of operation. See the Internal Oscillator section.
In FPWM mode, the converter switches at a constant
frequency with variable on-time. In skip mode, the con-
verter’s switching frequency is load-dependent until the
output load reaches a certain threshold. At higher load
current, the switching frequency does not change and the
operating mode is similar to the FPWM mode.
Figure 2. Current-Sense Configuration
DH1
OUTPUT SERIES RESISITOR SENSING
LX1
GND
CS1
OUT1
DL1
N
N
L1 R
CS
V
SUP
C
IN
C
OUT
MAX16993