MAX1644 Maxim, MAX1644 Datasheet - Page 8

• Manufacturer Part Number: MAX1644
• Description: Advanced Chemistry-Independent / Level 2 Battery Charger with Input Current Limiting
• Manufacturer: Maxim

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;
2) Select the constant-off-time as a function of input
3) Select R
4) Select the inductor as a function of 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
preset output voltage, connect FB to the output voltage,
and connect FBSEL to V
2A, Low-Voltage, Step-Down Regulator with
Synchronous Rectification and Internal Switches
Table 1. Recommended Component
Values (I
Table 2. Output Voltage and AC Load-
Regulation Selection
8
__________________Design Procedure
Unconnected
IN
300kHz is a good starting point.
voltage, output voltage, and switching frequency.
off-time, and peak-to-peak inductor current.
_______________________________________________________________________________________
FBSEL
with a 1% or 2% AC load-regulation error. For a
GND
V
V
REF
(V)
3.3
3.3
3.3
5
5
5
5
CC
IN
TOFF
OUT
PIN
as a function of off-time.
= 2A, f
Resistor
Resistor
Voltage
Voltage
Divider
Divider
V
Output
Output
(V)
3.3
2.5
1.8
1.5
2.5
1.8
1.5
OUT
FB
Setting the Output Voltage
PWM
CC
VOLTAGE
Adjustable
Adjustable
(2.5V output voltage) or
OUTPUT
(µH)
2.5
3.3
(V)
= 300kHz)
6.0
6.8
6.8
6.0
3.3
4.7
4.7
L
REGULATION
ERROR (%)
AC LOAD-
R
(kΩ)
120
180
240
270
180
200
TOFF
82
2
2
2
1
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
50kΩ for R1. R2 is given by the equation:
where V
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 to GND. R
time in PWM mode. Use the following equation to select
the off-time according to your desired switching fre-
quency in PWM mode (I
where: t
Figure 2. Adjustable Output Voltage
Programming the Switching Frequency
R1 = 50kΩ
R2 = R1(V
V
t
REF
OFF
REF
V
V
V
V
= 1.1V
OFF
IN
OUT
NMOS
PMOS
is typically 1.1V.
OUT
MAX1644
=
= the input voltage
= the programmed off-time
/ V
= the output voltage
R2
f
REF
PWM
TOFF
= the voltage drop across the internal
= the voltage drop across the internal
- 1)
PMOS power switch
NMOS synchronous-rectifier switch
(
=
V
IN
(
R1
V
sets the PMOS power switch off-
LX
FB
IN
OUT
–
⎛
⎜
⎝
−
V
V
V
OUT
REF
OUT
> 0.2A):
V
PMOS
TOFF
−
−
FB
1
V
⎞
⎟
⎠
+
, connected from
PMOS
equals V
and Off-Time
V
NMOS
)
REF
)
R2
R1
V
OUT
. Use