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MAX20002ATPB/V+

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P19
Spread-Spectrum Option
The spread spectrum can be enabled on the device
using a pin. When the SPS pin is pulled high the spread
spectrum is enabled and the operating frequency is varied
±3% centered on FOSC. The modulation signal is a trian-
gular wave with a period of 110μs at 2.2MHz. Therefore,
FOSC ramps down 3% and back to 2.2MHz in 110μs and
also ramps up 3% and back to 2.2MHz in 110μs. The
cycle repeats.
For operations at FOSC values other than 2.2MHz, the
modulation signal scales proportionally (e.g., at 400kHz,
the 110μs modulation period increases to 110μs x
2.2MHz/0.4MHz = 550μs).
The internal spread spectrum is disabled if the devices
are synchronized to an external clock. However, the
devices do not filter the input clock on the FSYNC pin and
pass any modulation (including spread spectrum) present
on the driving external clock.
Internal Oscillator (FOSC)
The switching frequency (f
SW
) is set by a resistor (R
FOSC
)
connected from FOSC to AGND. For example, a 400kHz
switching frequency is set with R
FOSC
= 73.2kΩ. Higher
frequencies allow designs with lower inductor values and
less output capacitance. Consequently, peak currents and
I
2
R losses are lower at higher switching frequencies, but
core losses, gate-charge currents, and switching losses
increase.
Overtemperature Protection
Thermal overload protection limits the total power
dissipation 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
converter, allowing the IC to cool. The thermal sensor
turns on the IC again after the junction temperature cools
by 15°C.
Overvoltage Protection (OVP)
If the output voltage reaches the OVP threshold, the
high-side switch is forced off and the low-side switch
is forced on until the negative-current limit is reached.
After negative-current limit is reached, both the high-side
and low-side switches are turned off. The MAX20002C
and MAX20003C feature an additional clamp and lower
OVP threshold to limit the output-voltage overshoot for
automotive conditions. Contact the Maxim Applications
team to determine if the MAX20002C/MAX20003C are
needed for your application.
Applications Information
Setting the Output Voltage
Connect FB to BIAS for a fixed +5V/3.3V output voltage.
To set the output to other voltages between 1V and 10V,
connect a resistive divider from output (OUT) to FB to
AGND (Figure 2). Select R
FB2
(FB to AGND resistor)
less than or equal to 500kΩ. Calculate R
FB1
(OUT to FB
resistor) with the following equation:
OUT
FB1 FB2
FB
V
R R -1
V


=





where VFB = 1V (see the
Electrical Characteristics
table).
Forced-PWM and Skip Modes
In PWM mode of operation, the devices switch at
a constant frequency with variable on-time. In skip
mode of operation, the converter’s switching frequency
is load dependent. At higher load current, the switching
frequency does not change and the operating mode is
similar to the PWM mode. Skip mode helps improve
efficiency in light-load applications by allowing the
converters to turn on the high-side switch only when the
output voltage falls below a set threshold. As such, the
converters do not switch MOSFETs on and off as often
as in the PWM mode. Consequently, the gate charge and
switching losses are much lower in skip mode.
Figure 2. Adjustable Output-Voltage Setting
R
FB1
R
FB2
V
OUT
MAX20002
MAX
20003
FB
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Maxim Integrated
│
13
MAX20002/MAX20003 36V, 220kHz to 2.2MHz, 2A/3A Fully
Integrated Step-Down Converters
with 15μA Operating Current
Inductor Selection
Three key inductor parameters must be specified for
operation with the devices: inductance value (L), inductor
saturation current (I
SAT
), and DC resistance (R
DCR
). To
select inductor value, the ratio of inductor peak-to-peak
AC current to DC average current (LIR) must be selected
first. A good compromise between size and loss is a 30%
peak-to-peak ripple current to average-current ratio (LIR
= 0.3). The switching frequency, input voltage, output volt-
age, and selected LIR then determine the inductor value
as follows:
SUP OUT OUT
SUP SW OUT
(V V ) V
L
V f I LIR
−×
=
×× ×
where V
SUP
, V
OUT
, and I
OUT
are typical values (so that
efficiency is optimum for typical conditions). The switch-
ing frequency is set by R
FOSC
(see TOC 8 in the Typical
Operating Characteristics section).
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor RMS current requirement (I
RMS
) is
defined by the following equation:
OUT SUP OUT
RMS LOAD(MAX)
SUP
V x(V - V )
II
V
= ×
I
RMS
has a maximum value when the input voltage
equals twice the output voltage:
SUP OUT
V 2V= ×
therefore:
LOAD(MAX)
RMS
SUP
I
I
V
=
Choose an input capacitor that exhibits less than +10°C
self-heating temperature rise at the RMS input current for
optimal long-term reliability.
The input-voltage ripple is comprised of ΔV
Q
(caused
by the capacitor discharge) and ΔV
ESR
(caused by the
ESR of the capacitor). Use low-ESR ceramic capacitors
with high ripple-current capability at the input. Assume
the contribution from the ESR and capacitor discharge
equal to 50%. Calculate the input capacitance and ESR
required for a specified input voltage ripple using the
following equations:
ESR
IN
L
OUT
V
ESR
I
I
2
∆
=
∆
+
where:
SUP OUT OUT
L
SUP SW
(V - V ) V
I
V fL
×
∆=
××
and:
OUT
IN
Q SW
I D(1 - D)
C
Vf
×
=
∆×
OUT
SUPSW
V
D
V
=
where: I
OUT
is the maximum output current and D is the
duty cycle.
Output Capacitor
The output filter capacitor must have low enough equiva-
lent series resistance (ESR) to meet output-ripple and
load-transient requirements. The output capacitance must
be high enough to absorb the inductor energy while
transitioning from full-load to no-load conditions without
tripping the overvoltage-fault protection. When using
high-capacitance, low-ESR capacitors, the filter capaci-
tor’s ESR dominates the output-voltage ripple, so the size
of the output capacitor depends on the maximum ESR
required to meet the output-voltage ripple (V
RIPPLE(P-P)
)
specifications:
RIPPLE(P-P) LOAD(MAX)
V ESR I LIR
=××
The actual capacitance value required relates to the
physical size needed to achieve low ESR, as well as
to the chemistry of the capacitor technology. Thus, the
capacitor is usually selected by ESR and voltage rating
rather than by capacitance value.
When using low-capacity filter capacitors, such as ceramic
capacitors, size is usually determined by the capacity need-
ed to prevent voltage droop and voltage rise from causing
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.
However, low-capacity filter capacitors typically have high-
ESR zeros that can affect the overall stability.
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Maxim Integrated
│
14
MAX20002/MAX20003 36V, 220kHz to 2.2MHz, 2A/3A Fully
Integrated Step-Down Converters
with 15μA Operating Current
Compensation Network
The devices use an internal transconductance error
amplifier with its inverting input and its output available to
the user for external frequency compensation. The output
capacitor and compensation network determine the loop
stability. The inductor and the output capacitor are chosen
based on performance, size, and cost. Additionally, the
compensation network optimizes the control-loop stability.
The converter uses a current-mode control scheme that
regulates the output voltage by forcing the required
current through the external inductor. The devices use
the voltage drop across the high-side MOSFET to sense
inductor current. Current-mode control eliminates the
double pole in the feedback loop caused by the inductor
and output capacitor, resulting in a smaller phase shift
and requiring less elaborate error-amplifier compensation
than voltage-mode control. Only a simple single series
resistor (R
C
) and capacitor (C
C
) are required to have a
stable, high-bandwidth loop in applications where ceramic
capacitors are used for output filtering (see Figure 3). For
other types of capacitors, due to the higher capacitance and
ESR, the frequency of the zero created by the capacitance
and ESR is lower than the desired closed-loop crossover
frequency. To stabilize a nonceramic output-capacitor loop,
add another compensation capacitor (C
F
) from COMP to
ground to cancel this ESR zero.
The basic regulator loop is modeled as a power modula-
tor, output feedback divider, and an error amplifier. The
power modulator has a DC gain set by g
m
× R
LOAD
,
with a pole and zero pair set by R
LOAD
, the output
capacitor (C
OUT
), and its ESR. The following equations
help to approximate the value for the gain of the power
modulator (GAIN
MOD(dc)
), neglecting the effect of the
ramp stabilization. Ramp stabilization is necessary when
the duty cycle is above 50% and is internally done for the
devices:
MOD(dc) mc LOAD
GAIN g R= ×
where R
LOAD
= V
OUT
/I
OUT(MAX)
in Ω and g
mc
= 3S.
In a current-mode step-down converter, the output capaci-
tor, its ESR, and the load resistance introduce a pole at
the following frequency:
pMOD
OUT LOAD
1
f
2C R
=
π× ×
The output capacitor and its ESR also introduce a zero at:
zMOD
OUT
1
f
2 ESR C
=
π× ×
When C
OUT
is composed of “n” identical capacitors
in parallel, the resulting C
OUT = n
× C
OUT(EACH)
, and
ESR = ESR(EACH)/n. Note that the capacitor zero for a
parallel combination of alike capacitors is the same as
for an individual capacitor.
The feedback voltage-divider has a gain of GAIN
FB
=
V
FB
/V
OUT
, where V
FB
is 1V (typ).
The transconductance error amplifier has a DC gain
of GAIN
EA(DC)
= g
m_EA
× R
OUT_EA
, where g
m_EA
is
the error amplifier transconductance, which is 700µS
(typ), and R
OUT_EA
is the output resistance of the error
amplifier (50MΩ).
Figure 3. Compensation Network
R
C
C
C
C
F
R1
R2
V
OUT
COMP
g
m
REF
www.maximintegrated.com
Maxim Integrated
│
15
MAX20002/MAX20003 36V, 220kHz to 2.2MHz, 2A/3A Fully
Integrated Step-Down Converters
with 15μA Operating Current
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P19

MAX20002ATPB/V+

Mfr. #:
Buy MAX20002ATPB/V+
Manufacturer:
Maxim Integrated
Description:
Switching Voltage Regulators 36V, 220kHz to 2.2MHz, 2A Step-Down Converter with 15uA Operating Current
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