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PDF ADP1864 Data sheet ( Hoja de datos )
| Número de pieza | ADP1864 | |
| Descripción | Constant Frequency Current-Mode Step-Down DC/DC Controller | |
| Fabricantes | Analog Devices | |
| Logotipo |  |
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Constant Frequency Current-Mode
Step-Down DC/DC Controller in TSOT
ADP1864
FEATURES
Wide input voltage range: 3.15 V to 14 V
Wide output voltage range: 0.8 V to input voltage
Pin-to-pin compatible with LTC1772, LTC3801
Up to 94% efficiency
0.8 V ±1.25% reference accuracy over temperature
Internal soft start
100% duty cycle for low dropout voltage
Current-mode operation for good line and load transient
response
7 μA shutdown current
235 μA quiescent supply current
Short-circuit and overvoltage protection
Small 6-lead TSOT package
APPLICATIONS
Wireless devices
1- to 3-cell Li-Ion battery-powered applications
Set-top boxes
Processor core power supplies
Hard disk drives
GENERAL DESCRIPTION
The ADP1864 is a compact, inexpensive, constant-frequency
current-mode step-down DC-to-DC controller. The ADP1864
drives a P-channel MOSFET that regulates an output voltage as
low as 0.8 V with ±2% accuracy, for up to 5 A load currents,
from input voltages as high as 14 V.
The ADP1864 provides system flexibility by allowing accurate
setting of the current limit with an external resistor, while the
output voltage is easily adjustable using two external resistors.
The ADP1864 includes an internal soft start to allow quick
power-up while preventing input inrush current. Additional
safety features include short-circuit protection, output over-
voltage protection, and input under voltage protection.
Current-mode control provides fast and stable load transient
performance, while the 580 kHz operating frequency allows a
small inductor to be used in the system. To further the life of a
battery source, the controller turns on the external P-channel
MOSFET 100% of the duty cycle in dropout.
The ADP1864 operates over the −40°C to +85°C temperature
range and is available in a small, low profile, 6-lead TSOT
package.
470pF
ADP1864
15kΩ
1 COMP
IN 5
2 GND
CS 4
80.6kΩ
3 FB
PGATE 6
174kΩ
0.03Ω
VIN = 3.3V to 10V
10μF
5μH 2.5V, 2.0A
47μF
Figure 1. Typical Applications Diagram
MOSFET = FDC638P
100
VOUT = 2.5V
95
90
85
3.3V
80
75
4.2V
70
65 5.0V
60
0.01
0.1 1.0
EFFICIENCY vs. LOAD CURRENT FOR APPLICATION
10.0
Figure 2. Efficiency vs. Load Current
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2005 Analog Devices, Inc. All rights reserved.
1 page  
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
ADP1864
COMP 1
6 PGATE
ADP1864
GND 2 TOP VIEW 5 IN
(Not to Scale)
FB 3
4 CS
Figure 3. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. Mnemonic Description
1
COMP
Regulator Compensation Node. COMP is the output of the internal transconductance error amplifier. Connect a
series RC from COMP to GND to compensate for the control loop. Add an extra high frequency capacitor between
COMP and GND to further reduce switching jitter. The value of this is typically one tenth of the main
compensation capacitor. Pulling the COMP pin below 0.3 V disables the ADP1864 and turns off the external PFET.
2 GND
Analog Ground. Directly connect the compensation and feedback networks to GND, preferably with a small
analog GND plane. Connect GND to the power ground (PGND) plane with a narrow track at a single point close to
the GND pin. See the Layout Considerations section for more information.
3 FB
Feedback Input. Connect a resistive voltage divider from the output voltage to FB to set the output voltage. The
regulation feedback voltage is 0.8 V. Place the feedback resistors as close as possible to the FB pin.
4 CS
Current Sense Input. CS is the negative input of the current sense amplifier. It provides the current feedback signal
used to terminate the PWM on-time. Place a current sense resistor between IN and CS to set the current limit. The
current limit threshold is typically 125 mV.
5 IN
Power Input. IN is the ADP1864 power supply and the positive input of the current sense amplifier. Connect IN
to the positive side of the input voltage source. Bypass IN to PGND with a 10 μF or larger capacitor, as close as
possible to the ADP1864. For additional high frequency noise reduction, add a 0.1 μF capacitor to PGND at the
IN pin.
6
PGATE
Gate Drive Output. PGATE drives the gate of the external P-channel MOSFET. Connect PGATE to the gate of the
external MOSFET.
Rev. 0 | Page 5 of 16
5 Page 
MOSFET
Choose the external P-channel MOSFET based on the following:
Vt (threshold voltage), maximum voltage and current ratings,
RDS(ON), and gate charge.
The minimum operating voltage of the ADP1864 is 3.15 V.
Choose a MOSFET with a Vt that is at least 1 V lower than the
minimum input supply voltage used in the application.
Ensure that the maximum ratings for MOSFET VGS and VDS are
a few volts greater than the maximum input voltage used with
the ADP1864.
Estimate the rms current in the MOSFET under continuous
conduction mode by
(( ))IMOSFET(RMS) =
⎜⎛
⎜⎝
VOUT + VD
VIN + VD
⎟⎞
⎟⎠
×
I LOAD
Derate the MOSFET current by at least 20% to account for
inductor ripple and changes in the diode voltage.
The MOSFET power dissipation is the sum of the conducted
and the switching losses:
( ) ( )PDMOSFET (COND) = I MOSFET(RMS) 2 × 1 +T × RDS(ON)
where T is 0.005/˚C × (MOSFET Junction Temperature − 25˚C).
Ensure the maximum power dissipation calculated is signif-
icantly less than the maximum rating of the MOSFET.
DIODE
The diode carries the inductor current during the off time of
the external FET. The average current of the diode is, therefore,
dependent on the duty cycle of the controller as well as the
output load current.
(( ))IDIODE(AV ) = ⎜⎜⎝⎛1 −
VOUT + VD
VIN + VD
⎟⎞
⎟⎠
×
I
LOAD
where VD is the diode forward drop. A typical Schottky diode
has a 0.5 V forward drop.
A Schottky diode is recommended for best efficiency because
it has a low forward drop and faster switching speed than
junction diodes. If a junction diode is used it must be an
ultrafast recovery diode. The low forward drop reduces the
power losses during the FET off time, and fast switching speed
reduces the switching losses during PFET transitions.
ADP1864
INPUT CAPACITOR
The input capacitor provides a low impedance path for the
pulsed current drawn by the external P-channel FET. Choose an
input capacitor whose impedance at the switching frequency is
lower than the impedance of the voltage source (VIN). The
preferred input capacitor is a 10μF ceramic capacitor due to its
low ESR and low impedance.
For all types of capacitors, make sure the ripple current rating of
the capacitor is greater than half of the maximum output load
current.
Where space is limited, multiple capacitors can be placed in
parallel to meet the rms current requirement. Place the input
capacitor as close as possible to the IN pin of the ADP1864.
OUTPUT CAPACITOR
The ESR and capacitance value of the output capacitor
determine the amount of output voltage ripple:
ΔV
≅
ΔI
×
⎜⎛
⎜⎝
8
×
f
1
× COUT
+
ESRCOUT
⎟⎞
⎟⎠
where f = oscillator frequency (typically 580 kHz).
Because the output capacitance is typically >40 μF, the ESR
dominates the voltage ripple. Ensure the output capacitor ripple
rating is greater than the maximum inductor ripple.
( ) ( )Irms
≅
1
2×
3
× ⎜⎜⎝⎛
VOUT
+ VD
L×
× VIN − VOUT
f × VIN
⎟⎟⎠⎞
POSCAP capacitors from Sanyo offer a good size, ESR, ripple,
and current capability trade-off.
Rev. 0 | Page 11 of 16
11 Page | ||
| Páginas | Total 16 Páginas | |
| PDF Descargar | [ Datasheet ADP1864.PDF ] | |
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