MAX1889ETE Maxim Integrated Products, MAX1889ETE Datasheet - Page 22

• Manufacturer Part Number: MAX1889ETE
• Description: DC/DC Switching Converters
• Manufacturer: Maxim Integrated Products

Specifications of MAX1889ETE

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

Triple-Output TFT LCD Power Supply
with Fault Protection
The transistor’s current gain limits the guaranteed maxi-
mum output current to:
where I
is the pullup resistor connected between the transis-
tor’s base and emitter. Furthermore, the transistor’s cur-
rent gain increases the linear regulator’s DC loop gain
(see the Stability Requirements section), so excessive
gain destabilizes the output. Therefore, transistors with
current gain over 100 at the maximum output current
are not recommended. The transistor’s input capaci-
tance and input resistance also create a second pole,
which could be low enough to make the output unsta-
ble when heavily loaded.
The transistor’s saturation voltage at the maximum output
current determines the minimum input-to-output voltage
differential that the linear regulator supports.
Alternatively, the package’s power dissipation could limit
the usable maximum input-to-output voltage differential.
The maximum power dissipation capability of the transis-
tor’s package and mounting must exceed the actual
power dissipation in the device. The power dissipation
equals the maximum load current times the maximum
input-to-output voltage differential:
Figure 7. Limiting LDO Drive Current During Startup
22
______________________________________________________________________________________
DRV
V
REF
V
NL
N
P I
I
LOAD MAX CE
is the minimum base-drive current, and R
I
LOAD MAX
=
LOAD MAX
(
(
(
Q1
)
V
)
=
)
(



V
I
DRV
LDOIN
-
R
V
-
BE
BE
V
LDOOUT



β
MIN
2200pF
15kΩ
C24
R19
)
=
7
6
DRVN
FBN
BE
MAX1889
During startup, the LDO outputs are below their respec-
tive setpoints, and the base drive to the pass transis-
tors is a maximum. The large drive currents can cause
the charge-pump outputs to collapse. If the charge-
pump loading is objectionable, base resistors can be
added between the drive outputs (DRVN and DRVP)
and the pass transistors (Figure 7). These resistors limit
the maximum drive current and prevent discharging the
charge pump’s output capacitors. Select the minimum
base drive current to meet the maximum required LDO
output current:
The resistance required to guarantee this base current is:
As a consequence of adding the base resistors, a volt-
age change at DRVN and DRVP accompanies changes
in drive current. This voltage change can be coupled
through parasitic capacitance to the LDO feedback
pins. If the rate of voltage change is sufficiently large, it
can cause instability.
DRVP
FBP
R
BASE
8
9
I
DRIVE MIN
≤
=
470pF
C20
51kΩ
R20
V
(
β
LDOIN MAX
MIN LDOIN MAX
I
DRIVE MIN
)
(
I
=
LDOOUT MAX
V
(
I
LDOOUT MAX
(
)
β
-
(
)
MIN
V
(
Q2
BE
(
)
)
-
V
)
BE
)
V
V
P
PL