14
FN6849.3
December 16, 2011
5 Power Conversion Functional Description
5.1 Internal Bias Regulators and Input
Supply Connections
The ZL2005P employs two internal low dropout
(LDO) regulators to supply bias voltages for internal
circuitry, allowing it to operate from a single input
supply. The internal bias regulators are as follows:
VR: The VR LDO provides a regulated 5V bias supply
for
the MOSFET driver circuits. It is powered
from the VDD pin and can supply up to 100 mA
output current. A 4.7 µF filter capacitor is
required at the VR pin.
V25: The V25 LDO provides a regulated 2.5V bias
supply for the main controller circuitry
. It is
powered from an internal 5V node and can sup-
ply up to 50 mA output current. A 10 µF filter
capac
itor is required at the V25 pin.
Note:
The internal bias regulators are designed for
powering internal circuitry only. Do not attach exter-
nal loads to any of these pins. The multi-mode pins
ma
y be connected to the VR or V25 pins for logic
HIGH settings.
When the input supply (V
DD
) is higher than 5.5V, the
VR pin should not be connected to any other pin. It
should only have a filter capacitor attached as shown
in Figure 9. Due to the dropout voltage associated with
the VR bias regulator, the VDD pin must be connected
to the V
R pin for designs operating from a VDD sup-
ply from 3.0V to 5.5V. Figure 9 illustrates the required
connections for both cases. F
or input supplies between
4.5V and 5.5V, either method can be used.
Figure 9. Input Supply Connections
5.2 High-side Driver Boost Circuit
The gate drive voltage for the upper MOSFET driver
is generated by a floating bootstrap capacitor, CB (see
Figure 3). When the lower MOSFET (QL) is turned
on, the SW node is pulled to
ground and the capacitor
is charged from the internal VR bias regulator through
diode DB. When QL turns off and the upper MOSFET
(QH) turns on, the SW node is pulled up to V
DD
and
the voltage on the BST pin is boosted approximately
5V above V
IN
to provide the necessary voltage for the
high-side driver. A Schottky diode should be used for
DB to maximize the high-side drive voltage.
5.3 Output Voltage Selection
Standard Mode (ZL2005)
The output voltage may be set to any voltage between
0.
6V and 5.0V provided that the input voltage is
higher than the desired output voltage by an amount
sufficient to prevent the device from exceeding its
maximum duty cycle specification. By connecting the
V0 and V1 pins to logic high, logic low, or leaving
them floating, V
OUT
can be set to any of nine standard
voltages as shown in Table 6.
Table 6.
Pin-strap Output Voltage Settings
If an output voltage other than those in Table 6 is
desired, the resistor setting method can
be used. Using
this method, resistors R0 and R1 are selected to pro-
duce a specific voltage between 0.6V and 5.0V in 10
mV steps.
Resistor R1 provides a coarse setting and
R0 a fine adjustment, thus eliminating the additional
errors associated with using two 1% resistors in a stan-
dard analog implementation (this ty
pically adds 1.4%
error using two 1% resistors).
To set V
OUT
using resistors, follow the steps below to
calculate an index value and then use Table 7 to select
the resistor that corresponds to the calculated index
value as
follows:
V0
LOW OPEN HIGH
V1
LOW
0.6V 0.8V 1.0V
OPEN
1.2V 1.5V 1.8V
HIGH
2.5V 3.3V 5.0V
ZL2005P
