MAX1644EAE-T Datasheet, PDF (8/11 Pages) Maxim Integrated Products – 2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches

2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
__________________Design Procedure
For typical applications, use the recommended compo-
nent values in Table 1. For other applications, take the
following steps:
1) Select the desired PWM-mode switching frequency;
300kHz is a good starting point.
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.
Table 1. Recommended Component
Values (I_OUT = 2A, f_PWM = 300kHz)
V_IN
V_OUT
L
R_TOFF
(V)
(V)
(µH)
(kΩ)
5
3.3
6.0
120
5
2.5
6.8
180
5
1.8
6.8
240
5
1.5
6.0
270
3.3
2.5
3.3
82
3.3
1.8
4.7
180
3.3
1.5
4.7
200
Table 2. Output Voltage and AC Load-
Regulation Selection
PIN
FBSEL
FB
OUTPUT
AC LOAD-
VOLTAGE REGULATION
(V)
ERROR (%)
V_CC
Output
Voltage
2.5
2
Unconnected
Output
Voltage
3.3
2
REF
Resistor
Divider
Adjustable
2
GND
Resistor
Divider
Adjustable
1
Setting the Output Voltage
The output of the MAX1644 is selectable between one
of two preset output voltages: (2.5V or 3.3V) with a 2%
AC load-regulation error, or an adjustable output volt-
age from the reference voltage (nominally 1.1V) up to
V_IN with a 1% or 2% AC load-regulation error. For a
preset output voltage, connect FB to the output voltage,
and connect FBSEL to V_CC (2.5V output voltage) or
LX
MAX1644
V_OUT
R2
FB
R1 = 50kΩ
R1
R2 = R1(V_OUT / V_REF - 1)
V_REF = 1.1V
Figure 2. Adjustable Output Voltage
leave unconnected (3.3V output voltage). Internal resis-
tor-dividers divide down the output voltage, regulating
the divided voltage to the internal reference voltage.
For output voltages other than 2.5V or 3.3V, or for
tighter AC load regulation, connect FBSEL to GND (1%
regulation) or to REF (2% regulation), and connect FB
to a resistor divider between the output voltage and
ground (Figure 2). Regulation is maintained for
adjustable output voltages when V_FB equals V_REF. Use
50kΩ for R1. R2 is given by the equation:
R2
=
R1
⎛
⎝⎜
V_OUT
V_REF
⎞
− 1_⎠⎟
where V_REF is typically 1.1V.
Programming the Switching Frequency
and Off-Time
The MAX1644 features a programmable PWM mode
switching frequency, which is set by the input and out-
put voltage and the value of R_TOFF, connected 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 fre-
quency in PWM mode (I_OUT > 0.2A):
( ₍ ) _{) tOFF}
=
V_IN – V_OUT − V_PMOS
f_PWM V_IN − V_PMOS + V_NMOS
where:
t_OFF = the programmed off-time
V_IN = the input voltage
V_OUT = the output voltage
V_NMOS = the voltage drop across the internal
PMOS power switch
V_PMOS = the voltage drop across the internal
NMOS synchronous-rectifier switch
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