MAX1649CPA Maxim Integrated Products, MAX1649CPA Datasheet - Page 9

• Manufacturer Part Number: MAX1649CPA
• Description: IC CTRLR DCDC 3V/ADJ LDO 8-DIP
• Manufacturer: Maxim Integrated Products
• Type: Step-Down (Buck)r

Specifications of MAX1649CPA

• Internal Switch(s): No
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V, 1.5 ~ 16 V
• Current - Output: 2.5A
• Frequency - Switching: Up to 350kHz
• Voltage - Input: 3 ~ 16 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)
• Power - Output: 727mW
• Output Voltage: 5 V, 1.5 V to 16 V
• Output Current: 2.5 A
• Input Voltage: 3 V to 16 V
• Mounting Style: Through Hole
• Maximum Operating Temperature: + 70 C
• Minimum Operating Temperature: 0 C
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

To maximize efficiency and reduce the size and cost
of external components, minimize the peak current.
However, since the available output current is a func-
tion of the peak current, the peak current must not be
too low.
To choose the proper current-sense resistor for a par-
ticular output voltage, determine the minimum input
voltage and the maximum load current. Next, refer-
ring to Figures 5a or 5b, using the minimum input volt-
age, find the curve with the largest sense resistor that
provides sufficient output current. It is not necessary
to perform worst-case calculations. These curves take
into account the sense-resistor (±5%) and inductor
(47µH ±10%) values, the diode drop (0.4), and the
IC’s current-sense trip level (85mV); an external MOS-
FET on-resistance of 0.07Ω is assumed for V
Standard wire-wound and metal-film resistors have an
inductance high enough to degrade performance.
Surface-mount (chip) resistors have very little inductance
and are well suited for use as current-sense resistors.
A U-shaped wire resistor made by IRC works well in
through-hole applications. Because this resistor is a
band of metal shaped as a “U”, its inductance is less
than 10nH (an order of magnitude less than metal film
resistors). Resistance values between 5mΩ and 0.1Ω
are available (see Table 1).
The MAX1649/MAX1651 operate with a wide range of
inductor values, although for most applications coils
between 10µH and 68µH take best advantage of the con-
Figure 5a. MAX1649 Current-Sense Resistor Graph
3.0
2.5
2.0
1.5
1.0
0.5
Low-Dropout, Step-Down DC-DC Controllers
0
5.0
V
OUT
= 5V
5.4
_______________________________________________________________________________________
INPUT VOLTAGE (V)
5V/3.3V or Adjustable, High-Efficiency,
5.8
r
r
r
r
r
r
6.2
s
s
s
s
s
s
= 0.030
= 0.040
= 0.050
= 0.060
= 0.080
= 0.100
Inductor Selection
6.6
16.0
GS
= -5V.
trollers’ high switching frequency. With a high inductor
value, the MAX1649/MAX1651 will begin continuous-cur-
rent operation (see Detailed Description) at a lower frac-
tion of full-load current. In general, smaller values pro-
duce higher ripple (see below) while larger values require
larger size for a given current rating.
In both the continuous and discontinuous modes, the
lower limit of the inductor is important. With a too-small
inductor value, the current rises faster and overshoots the
desired peak current limit because the current-limit com-
parator has a finite response time (300ns). This reduces
efficiency and, more importantly, could cause the current
rating of the external components to be exceeded.
Calculate the minimum inductor value as follows:
where ΔI is the inductor-current overshoot factor,
I
parator to switch. Set ΔI = 0.1 for an overshoot of 10%.
For highest efficiency, use a coil with low DC resis-
tance; a value smaller than 0.1V/I
minimize radiated noise, use a toroid, pot core, or
shielded-bobbin inductor. Inductors with a ferrite core
or equivalent are recommended. Make sure the induc-
tor’s saturation-current rating is greater than I
However, it is generally acceptable to bias the inductor
into saturation by about 20% (the point where the
inductance is 20% below its nominal value).
Figure 5b. MAX1651 Current-Sense Resistor Graph
LIM
= V
CS
L(min) = ——————————––——
/R
3.0
2.5
2.0
1.5
1.0
0.5
SENSE
0
3.0
V
OUT
, and 0.3µs is the time it takes the com-
= 3.3V
3.4
(V+(max) - V
INPUT VOLTAGE (V)
3.8
ΔI x I
r
r
r
r
r
r
4.2
s
s
s
s
s
s
= 0.030
= 0.040
= 0.050
= 0.060
= 0.080
= 0.100
OUT
LIM
LIM
) x 0.3µs
4.6
works best. To
16.0
LIM
(max).
9