Datasheet

High-Voltage, 2.2MHz, 2A Automotive Step-
Down Converter with Low Operating Current
MAX16974
______________________________________________________________________________________ 15
When using low-capacity filter capacitors, such as
ceramic capacitors, size is usually determined by the
capacity needed to prevent V
SAG
and V
SOAR
from caus-
ing problems during load transients. Generally, once
enough capacitance is added to meet the overshoot
requirement, undershoot at the rising-load edge is no
longer a problem (see the V
SAG
and V
SOAR
equations
in the Transient Response section). However, low-
capacity filter capacitors typically have high-ESR zeros
that can affect the overall stability. Other important crite-
ria in the choice of the total output capacitance are the
device’s soft-start time and maximum current capability
(see the Soft-Start Time and Maximum Allowed Output
Capacitance section).
Soft-Start Time and Maximum
Allowed Output Capacitance
The device’s soft-start time depends on the selected
switching frequency. The soft-start time is fixed to 2048
cycles, regardless of the switching frequency. This
means at 2.2MHz the soft-start time is ~0.93ms, and at
220kHz the soft-start time is ~9.3ms.
The device is a 2A-capable switching regulator and the
amount of load present at startup determines the total
output capacitance allowed for a particular application.
OUT(MAX) SW
OUT LX(MIN) LOAD(MAX)
C 2048/f
1/ V I - I
≈ ×
 
∆ ×
 
Keeping the above equation in mind, see the following
table to ensure that C
OUT
is less than maximum allowed
values.
Transient Response
The inductor ripple current also impacts transient
response performance, especially at low V
SUP
- V
OUT
differentials. Low inductor values allow the inductor cur-
rent to slew faster, replenishing charge removed from the
output filter capacitors by a sudden load step. The total
output-voltage sag is the sum of the voltage sag while
the inductor is ramping up and the voltage sag before
the next pulse can occur:
( )
( )
( )
( )
2
LOAD(MAX)
LOAD(MAX)
SAG
OUT
OUT SUP MAX OUT
L I
I t t
V
C
2C V D V
∆
∆ − ∆
= +
× −
where D
MAX
is the maximum duty factor (see the
Electrical Characteristics table), L is the inductor value
in FH, C
OUT
is the output capacitor value in FF, t is the
switching period (1/f
SW
) in Fs, and δt equals (V
OUT
/
V
SUP
x t when in fixed-frequency PWM mode, or L x 0.2
x I
MAX
/(V
SUP
- V
OUT
) when in skip mode. The amount of
overshoot (V
SOAR
) during a full-load to a no-load tran-
sient due to stored inductor energy can be calculated
as:
( )
( )
2
SOAR LOAD(MAX) OUT OUT
V I L/ 2 x C V≈ ∆ × ×
Rectifier Selection
The device requires an external Schottky diode rectifier
as a freewheeling diode. Connect this rectifier close
to the device using short leads and short PCB traces.
Choose a rectifier with a continuous current rating greater
than the highest output current-limit threshold (3.5A), and
with a voltage rating greater than the maximum expected
input voltage, V
SUPSW
. Use a low forward-voltage-drop
Schottky rectifier to limit the negative voltage at LX. Avoid
higher than necessary reverse-voltage Schottky rectifiers
that have higher forward-voltage drops.
Compensation Network
The device uses an internal transconductance error
amplifier with its inverting input and output available
to the user for external frequency compensation. The
output capacitor and compensation network determine
the loop stability. The inductor and the output capaci-
tor are chosen based on performance, size, and cost.
Additionally, the compensation network optimizes the
control-loop stability.
The controller uses a current-mode control scheme that
regulates the output voltage by forcing the required cur-
rent through the external inductor, so the device uses
FREQUENCY = 400kHz
V
OUT
(V)
I
LOAD
(STARTUP) (A)
C
OUT
(MAX ALLOWED)
3.3 2
775FF
5 2
512FF
3.3 0 3.9mF
5 0 2.6mF
FREQUENCY = 2.2MHz
V
OUT
(V)
I
LOAD
(STARTUP) (A)
C
OUT
(MAX ALLOWED)
3.3 2
140FF
5 2
93FF
3.3 0
705FF
5 0
465FF