Datasheet

High-Voltage, 2.2MHz, 2A Automotive Step-
Down Converter with Low Operating Current
MAX16974
12 _____________________________________________________________________________________
Skip Mode
During light-load operation, I
INDUCTOR
P 240mA, the
device enters skip-mode operation. Skip mode turns
off the internal switch and allows the output to drop
below regulation voltage before the switch is turned on
again. The lower the load current, the longer it takes for
the regulator to initiate a new cycle. Because the con-
verter skips unnecessary cycles, the converter efficiency
increases. During skip mode the quiescent current drops
to 35FA.
Overtemperature Protection
Thermal-overload protection limits the total power dissipa-
tion in the device. When the junction temperature exceeds
+175°C (typ), an internal thermal sensor shuts down the
internal bias regulator and the step-down controller, allow-
ing the IC to cool. The thermal sensor turns on the device
again after the junction temperature cools by +15°C.
Applications Information
Output Voltage/Reset Threshold
Resistive Divider Network
Although the device’s output voltage and reset thresh-
old can be set individually, Figure 3 shows a combined
resistive divider network to set the desired output voltage
and the reset threshold using three resistors. Use the
following formula to determine the R
FB3
of the resistive
divider network:
TOTAL REF
FB3
OUT
R V
R
V
×
=
where V
REF
= 1V, R
TOTAL
= selected total resistance of
R
FB1
, R
FB2
, and R
FB3
in ohms, and V
OUT
is the desired
output voltage in volts.
TOTAL REF_RES
FB2 FB3
RES
R V
R - R
V
×
=
where V
REF_RES
is 1.2V (see the Electrical Characteristics
table), and V
RES
is the desired reset threshold in volts.
The precision of the reset threshold function is dependent
on the tolerance of the resistors used for the divider. Care
must be taken to choose the values of the resistors. Too
small a resistor value adds to the device’s quiescent cur-
rent, whereas if the resistors are too large, there is some
noise susceptibility to the FB pin.
Boost Capacitor for
Dropout Operation
The device has an internal boost capacitor refresh algo-
rithm for dropout operation. This is required to ensure
proper boost capacitor voltage, which delivers power to
the gate drive circuitry. If the high-side MOSFET is on
consecutively for 3.65 clock cycles, the internal counter
detects this and turns off the high-side MOSFET for 0.35
clock cycles. This is of particular concern when V
IN
is
falling and approaching V
OUT
and a minimum switching
frequency of 220kHz is used.
The worst-case condition for boost capacitor refresh time
is with no load on the output. For the boost capacitor
to recharge completely, the LX node must be pulled to
ground. If there is no current in the inductor, the LX node
does not go to ground. To solve this issue, an internal
load of approximately 100mA is turned on at the 6th
clock cycle, which is determined by a separate counter.
In the worst-case condition with no load, the LX node
does not go below ground during the first detect of the
3.65 clock cycles. It must wait for the next 3.65 clock
cycles to finish. This means the soonest the LX node can
go below ground is 4 + 3.65 = 7.65 clock cycles. This
time does not factor in the size of the inductor and the
time it takes for the inductor current to build up to 100mA
(internal load).
So no-load minimum time before refresh is:
dt (no load) = 7.65 clock cycles = 7.65 x 5µs
(at 220kHz) = 34.77µs
Figure 3. Output Voltage/Reset Threshold Resistive Divider
Network
MAX16974
R
FB3
R
FB2
R
FB1
V
OUT
RESETI
FB