ADP3026 データシートの表示（PDF） - Analog Devices
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ADP3026 Datasheet PDF : 20 Pages
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ADP3026
THEORY OF OPERATION
The ADP3026 is a step-down power supply controller for
battery-powered applications. The ADP3026 contains the
control circuit for two synchronous step-down converter. for
fixed 3.3 V and 5 V outputs.
CIRCUIT DESCRIPTION
Internal 5 V Supply (INTVCC)
An internal low dropout regulator (LDO) generates a 5 V
supply (INTVCC) to power all of the functional blocks within
the IC. The total current rating of this LDO is 50 mA. However,
this current is used for supplying gate-drive power, and it is
recommended that current is not drawn from this pin for other
purposes. Bypass INTVCC to AGND with a 4.7 µF capacitor. A
UVLO circuit is also included in the regulator. When INTVCC
< 4.1 V, the two switching regulators and the linear regulator
controller are shut down. The UVLO hysteresis voltage is about
270 mV. The internal LDO has a built-in foldback current limit
so that it will be protected if a short circuit is applied to the 5 V
output.
Reference (REF)
The ADP3026 contains a precision 800 mV band gap reference.
Bypass REF to AGND with a 22 nF ceramic capacitor. The
reference is for internal use only; do not draw current from REF.
Boosted High-Side Gate Drive Supply (BST)
The gate drive voltage for the high-side N-channel MOSFET is
generated by a flying capacitor boost circuit. The boost
capacitor connected between BST and SW is charged from the
INTVCC supply. Use only small-signal diodes for the boost
circuit.
Synchronous Rectifier (DRVL)
Synchronous rectification is used to improve efficiency, reduce
conduction losses, and ensure proper start-up of the boost gate
driver circuit. Antishoot-through protection is included to
prevent cross conduction during switch transitions. The low-
side driver must be turned off before the high-side driver is
turned on. For typical N-channel MOSFETs, the dead time is
about 50 ns. On the other edge, a dead time of about 50 ns is
achieved by an internal delay circuit. The synchronous rectifier
is turned off when the current flowing through the low-side
Table 4. Operating Modes
SD
SS5
Low
X
High
SS5 < 0.6 V
High
0.6 V < SS5 < 1.8 V
High
1.8 V < SS5
High
X
High
X
SS3
X
SS3 < 0.6 V
X
X
0.6 V < SS3 < 1.8 V
1.8 V < SS3
MOSFET falls to zero when in discontinuous conduction mode
(DCM). In continuous conduction mode (CCM), the current
flowing through the low-side MOSFET never reaches zero, so
the synchronous rectifier is turned off by the next clock cycle.
Shutdown (SD)
Holding SD low puts the ADP3026 into ultralow current
shutdown mode. For automatic startup, tie SD to VIN through a
resistor.
Soft Start and Power-Up Sequencing (SS)
SS3 and SS5 are soft start pins for the two controllers. A 2.1 µA
pull-up current charges an external soft start capacitor. Power-
up sequencing is easily done by choosing different capacitance.
When SS3/SS5 < 0.6 V, the two switching regulators are turned
off. When 0.6 V < SS5/SS3 < 1.8 V, the regulators start working
in soft start mode. When SS3/SS5 > 1.8 V, the regulators are in
normal operating mode. The minimum soft start time (~20 µs)
is set by an internal capacitor. Table 4 shows the ADP3026’s
operating modes.
Current Limiting (CLSET)
A cycle-by-cycle current limiting scheme is used by monitoring
current through the top N-channel MOSFET when it is turned
on. By measuring the voltage drop across the high-side
MOSFET VDS(ON), the external sense resistor is not required. The
current limit value is controlled by CLSET. When CLSET is
floating, the maximum VDS(ON) = 72 mV at room temperature;
when CLSET = 0 V, the maximum VDS(ON) = 300 mV at room
temperature. An external resistor (REXT) between CLSET and
AGND sets a current limit value between 72 mV and 300 mV.
The relationship between the external resistance and the
maximum VDS(ON) is
V
DS(ON )MAX
=
72
mV
(110 kΩ + REXT)
(26 kΩ + REXT)
(1)
The temperature coefficient of R DS(ON) of the N-channel
MOSFET is canceled by the internal current limit circuitry so
that the current limit value is accurate over a wide temperature
range.
Description
All Circuits Turned Off
5 V and 3.3 V Off; INTVCC = 5 V, REF = 0.8 V
5 V in Soft Start
5 V in Normal Operation
3.3 V in Soft Start
3.3 V in Normal Operation
Rev. 0 | Page 10 of 20
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