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MAX1742EEE+T

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
10 ______________________________________________________________________________________
Synchronous Rectification
In a step-down regulator without synchronous rectifica-
tion, an external Schottky diode provides a path for cur-
rent to flow when the inductor is discharging. Replacing
the Schottky diode with a low-resistance NMOS syn-
chronous switch reduces conduction losses and
improves efficiency.
The NMOS synchronous-rectifier switch turns on follow-
ing a short delay after the PMOS power switch turns off,
thus preventing cross conduction or “shoot through.” In
V
CC
470pF
2.2μF
1μF
10μF
10Ω
FBSEL
0.01μF
FEEDBACK
SELECTION
CURRENT
SENSE
PWM LOGIC
AND
DRIVERS
SS
IN
FB
V
IN
3.0V TO 5.5V
LX
PGNDTOFF
R
TOFF
GND
NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.
REF
REF
SUMMING
COMPARATOR
REF
REF
COMP
SKIP
SHDN
TIMER
V
IN
CURRENT
SENSE
G
m
C
OUT
V
OUT
MAX1742
MAX1842
Figure 2. Functional Diagram
TOFF
COMP
V
CC
FBSEL
SHDN
IN
PGND
GND
REF
SS
LX
FB
MAX1742
R
TOFF
1μF
0.01μF
L
2.2μF
470pF
10Ω
INPUT
C
IN
= 10μF (MAX1742)
C
IN
= 33μF (MAX1842)
OUTPUT
C
OUT
= 47μF (MAX1742)
C
OUT
= 150μF (MAX1842)
V
OUT
= 2.5V, FBSEL = V
CC
V
OUT
= 1.8V, FBSEL = REF
V
OUT
= 1.5V, FBSEL = FLOATING
Figure 1. Typical Circuit
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
______________________________________________________________________________________ 11
constant-off-time mode, the synchronous-rectifier
switch turns off just prior to the PMOS power switch
turning on. While both switches are off, inductor current
flows through the internal body diode of the NMOS
switch. The internal body diode’s forward voltage is rel-
atively high.
Thermal Resistance
Junction-to-ambient thermal resistance, θ
JA
, is highly
dependent on the amount of copper area immediately
surrounding the IC leads. The MAX1742 evaluation kit
has 0.5in
2
of copper area and a thermal resistance of
80°C/W with no forced airflow. Airflow over the board
significantly reduces the junction-to-ambient thermal
resistance. For heatsinking purposes, evenly distribute
the copper area connected at the IC among the high-
current pins.
Power Dissipation
Power dissipation in the MAX1742/MAX1842 is domi-
nated by conduction losses in the two internal power
switches. Power dissipation due to supply current in the
control section and average current used to charge
and discharge the gate capacitance of the internal
switches (i.e., switching losses) is approximately:
P
DS
= C x V
IN
2
x f
PWM
where C = 2.5nF and f
PWM
is the switching frequen-
cy in PWM mode.
This number is reduced when the switching frequency
decreases as the part enters Idle Mode. Combined con-
duction losses in the two power switches are approxi-
mated by:
P
D
= I
OUT
2
x R
PMOS
where R
PMOS
is the on-resistance of the PMOS switch.
The junction-to-ambient thermal resistance required to
dissipate this amount of power is calculated by:
θ
JA
= (T
J,MAX
- T
A,MAX
) / P
D(T
OT
)
where: θ
JA
= junction-to-ambient thermal resistance
T
J,MAX
= maximum junction temperature
T
A,MAX
= maximum ambient temperature
P
D(TOT)
= total losses
__________________Design Procedure
For typical applications, use the recommended compo-
nent values in Tables 1 or 2. For other applications,
take the following steps:
1) Select the desired PWM-mode switching frequency;
1MHz is a good starting point. See Figure 3 for maxi-
mum operating frequency.
V
OUT
(V)
R
TOFF
(kΩ)
5.6 39
L
(μH)
5 3.3
5.6
V
IN
(V)
47
5.6 755 1.8
3.9 393.3 2.5
3.9 433.3 1.8
3.9 563.3 1.5
5 2.5
Table 1. MAX1742 Recommended
Component Values (I
OUT
= 1A)
5.6 1005 1.5
f
PWM
(kHz)
850
910
610
1050
1000
770
1070
Table 2. MAX1842 Recommended
Component Values (Continuous Output
Current = 1A, Burst Output Current = 2.7A)
1180
715
940
985
570
850
800
f
PWM
(kHz)
1.55 1002.2
2.55
1.53.3 561.5
1.83.3 431.5
2.53.3 391.5
1.85 752.2
47
V
IN
(V)
2.2
3.35
L
(μH)
392.2
R
TOFF
(kΩ)
V
OUT
(V)
0
400
200
800
600
1200
1000
1400
2.6 3.6 4.13.1 4.6 5.1 5.6
MAXIMUM RECOMMENDED
OPERATING FREQUENCY vs. INPUT VOLTAGE
MAX1842 fig03
V
IN
(V)
OPERATING FREQUENCY (kHz)
V
OUT
= 1.5V
V
OUT
= 1.8V
V
OUT
= 2.5V
V
OUT
= 3.3V
Figure 3. Maximum Recommended Operating Frequency vs.
Input Voltage
MAX1742/MAX1842
1A/2.7A, 1MHz, Step-Down Regulators with
Synchronous Rectification and Internal Switches
12 ______________________________________________________________________________________
2) Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.
3) Select R
TOFF
as a function of off-time.
4) Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Setting the Output Voltage
The output of the MAX1742/MAX1842 is selectable
between one of three preset output voltages: 2.5V,
1.8V, and 1.5V. For a preset output voltage, connect FB
to the output voltage and connect FBSEL as indicated
in Table 3. For an adjustable output voltage, connect
FBSEL to GND and connect FB to a resistive divider
between the output voltage and ground (Figure 4).
Regulation is maintained for adjustable output voltages
when V
FB
= V
REF
. Use 50kΩ for R1. R2 is given by the
equation:
where V
REF
is typically 1.1V.
Programming the Switching Frequency
and Off-Time
The MAX1742/MAX1842 features a programmable
PWM mode switching frequency, which is set by the
input and output voltage and the value of R
TOFF
, con-
nected from TOFF to GND. R
TOFF
sets the PMOS
power switch off-time in PWM mode. Use the following
equation to select the off-time according to your
desired switching frequency in PWM mode:
where: t
OFF
= the programmed off-time
V
IN
= the input voltage
V
OUT
= the output voltage
V
PMOS
= the voltage drop across the internal
PMOS power switch
V
NMOS
= the voltage drop across the internal
NMOS synchronous-rectifier switch
f
PWM
= switching frequency in PWM mode
Select R
TOFF
according to the formula:
R
TOFF
= (t
OFF
- 0.07µs) (110kΩ / 1.00µs)
Recommended values for R
TOFF
range from 36kΩ to
430kΩ for off-times of 0.4µs to 4µs.
Inductor Selection
The key inductor parameters must be specified: inductor
value (L) and peak current (I
PEAK
). The following equa-
tion includes a constant, denoted as LIR, which is the
ratio of peak-to-peak inductor AC current (ripple current)
to maximum DC load current. A higher value of LIR allows
smaller inductance but results in higher losses and ripple.
A good compromise between size and losses is found at
approximately a 25% ripple-current to load-current ratio
(LIR = 0.25), which corresponds to a peak inductor cur-
rent 1.125 times the DC load current:
where: I
OUT
= maximum DC load current
LIR = ratio of peak-to-peak AC inductor current
to DC load current, typically 0.25
L
Vt
I LIR
OUT OFF
OUT
=
×
×
t
VV V
fVV V
OFF
IN OUT PMOS
PWM IN PMOS NMOS
–
=
−
()
−+
()
R2 R1
V
V
1
OUT
REF
=−
⎛
⎝
⎜
⎞
⎠
⎟
Figure 4. Adjustable Output Voltage
PIN
2.5V
CC
Output voltage
1.5
1.8REF Output voltage
AdjustableGND
Resistive
divider
FB
OUTPUT
VOLTAGE
(V)
FBSEL
Unconnected Output voltage
Table 3. Output Voltage Programming
LX
R2
R1
R1 = 50kΩ
R2 = R1(V
OUT
/ V
REF
- 1)
V
REF
= 1.1V
FB
V
OUT
MAX1742
MAX1842
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

MAX1742EEE+T

Mfr. #:
Buy MAX1742EEE+T
Manufacturer:
Maxim Integrated
Description:
Switching Voltage Regulators 1A/2.7A 1MHz Step Down Regulator
Lifecycle:
New from this manufacturer.
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