Datasheet

ADP1850 Data Sheet
Rev. A | Page 14 of 32
SYNCHRONOUS RECTIFIER AND DEAD TIME
The synchronous rectifier (low-side MOSFET) improves efficiency
by replacing the Schottky diode that is normally used in an
asynchronous buck regulator. In the ADP1850, the antishoot-
through circuit monitors the SW and DL nodes and adjusts the
low-side and high-side drivers to ensure break-before-make
switching which prevents cross-conduction or shoot-through
between the high-side and low-side MOSFETs. This break-
before-make switching is known as dead time, which is not
fixed and depends on how fast the MOSFETs are turned on
and off. In a typical application circuit that uses medium sized
MOSFETs with input capacitance of approximately 3 nF, the
typical dead time is approximately 30 ns. When small and fast
MOSFETs with fast diode recovery time are used, the dead time
can be as low as 13 ns.
INPUT UNDERVOLTAGE LOCKOUT
When the bias input voltage, V
IN
, is less than the undervoltage
lockout (UVLO) threshold, the switch drivers stay inactive.
When V
IN
exceeds the UVLO threshold, the switchers start
switching.
INTERNAL LINEAR REGULATOR
The internal linear regulator is low dropout (LDO) meaning it
can regulate its output voltage, VCCO. VCCO powers up the
internal control circuitry and provides power for the gate
drivers. It is guaranteed to have more than 200 mA of output
current capability, which is sufficient to handle the gate drive
requirements of typical logic threshold MOSFETs driven at up
to 1.5 MHz. VCCO is always active and cannot be shut down by
the EN1 and EN2 pins. Bypass VCCO to AGND with a 1 µF or
greater capacitor.
Because the LDO supplies the gate drive current, the output of
VCCO is subject to sharp transient currents as the drivers
switch and the boost capacitors recharge during each switching
cycle. The LDO has been optimized to handle these transients
without overload faults. Due to the gate drive loading, using the
VCCO output for other external auxiliary system loads is not
recommended.
The LDO includes a current limit well above the expected
maximum gate drive load. This current limit also includes a
short-circuit fold back to further limit the VCCO current in the
event of a short-circuit fault.
The VDL pin provides power to the low-side driver. Connect
VDL to VCCO. Bypass VDL to PGNDx with a 1 µF (minimum)
ceramic capacitor, which must be placed close to the VDL pin.
For an input voltage less than 5.5 V, it is recommended to
bypass the LDO by connecting VIN to VCCO, as shown in
Figure 26, thus eliminating the dropout voltage. However, if
the input range is 4 V to 7 V, the LDO cannot be bypassed by
shorting VIN to VCCO because the 7 V input has exceeded the
maximum voltage rating of the VCCO pin. In this case, use the
LDO to drive the internal drivers, but keep in mind that there is
a dropout when V
IN
is less than 5 V.
V
IN
= 2.75V TO 5.5V
ADP1850
VIN VCCO
09440-027
Figure 26. Configuration for V
IN
< 5.5 V
OVERVOLTAGE PROTECTION
The ADP1850 has a built-in circuit for detecting output over-
voltage at the FB node. When the FB voltage, V
FB
, rises above
the overvoltage threshold, the low-side N-channel MOSFET
(NMOSFET) is immediately turned on, and the high-side
NMOSFET is turned off until the V
FB
drops below the
undervoltage threshold. This action is known as the crow-
bar overvoltage protection. If the overvoltage condition is
not removed, the controller maintains the feedback voltage
between the overvoltage and undervoltage thresholds, and the
output is regulated to within typically +8% and −8% of the
regulation voltage. During an overvoltage event, the SS node
discharges toward zero through an internal 3 kΩ pull-down
resistor. When the voltage at FBx drops below the undervoltage
threshold, the soft start sequence restarts. Figure 27 shows the
overvoltage protection scheme in action in PSM.
CH1 20.0V CH2 5.00V
CH3 1.00V CH4 10.0V
M100µs A CH1 10.0V
1
2
4
3
DH1
PGOOD1
VO1 = 1.8V SHORTED TO 2V SOURCE
VIN
09440-028
Figure 27. Overvoltage Protection in PSM
POWER GOOD
The PGOODx pin is an open-drain NMOSFET with an internal
12 kΩ pull-up resistor connected between PGOODx and VCCO.
PGOODx is internally pulled up to VCCO during normal
operation and is active low when tripped. When the feedback
voltage, V
FB
, rises above the overvoltage threshold or drops
below the undervoltage threshold, the PGOODx output is
pulled to ground after a delay of 12 µs. The overvoltage or
undervoltage condition must exist for more than 10 µs for
PGOODx to become active. The PGOODx output also
becomes active if a thermal overload condition is detected.