LTC3450 Datasheet, PDF (8/12 Pages) Linear Technology – Triple Output Power Supply for Small TFT-LCD Displays

LTC3450
U
OPERATIO
The LTC3450 is a highly integrated power converter in-
tended for small TFT-LCD display modules. A fixed fre-
quency, synchronous PWM boost regulator generates a
low noise 5.1V, 10mA bias at greater than 90% efficiency
from an input voltage of 1.5V to 4.6V. Three charge pump
converters use the 5.1V output to generate 10V, 15V and
–5V, –10V or –15V at load currents up to 500µA. Each
converter is frequency synchronized to the main 500kHz
(nominal) boost converter. The generated output voltages
are internally sequenced to insure proper initialization of
the LCD panel. A digital shutdown input rapidly discharges
each generated output voltage to provide a near instanta-
neous turn-off of the LCD display.
Boost Converter
The synchronous boost converter utilizes current mode
control and includes internally set control loop and slope
compensation for optimized performance and simple de-
sign. Only three external components are required to
complete the design of the 5.1V, 10mA boost converter.
The high operation frequency produces very low output
ripple and allows the use of small low profile inductors and
tiny external ceramic capacitors. The boost converter also
disconnects its output from V_IN during shutdown to avoid
loading the input power source. Softstart produces a
controlled ramp of the converter input current during
startup, reducing the burden on the input power source.
Very low operating quiescent current and synchronous
operation allow for greater than 90% conversion effi-
ciency.
The MODE input reduces the boost converter operating
frequency by approximately 8x when driven high and
reduces the output power capability of the boost con-
verter. MODE is asserted when the polysilicon TFT-LCD
display is in its extremely low power blank condition. The
boost converter further reduces its quiescent current in
this mode, delivering both lower input (battery) current
drain and low noise operation.
Charge Pumps
The LTC3450 includes three separate charge pump con-
verters which generate 10V, 15V and either –5V, –10V or
–15V. Each output can deliver a maximum of 500µA. The
charge pumps feature fixed frequency, open-loop opera-
tion for high efficiency and lowest noise performance. The
charge pump converters operate at 1/8 the boost con-
verter frequency and include internal charge transfer
switches. Thus, each charge pump requires only two small
external capacitors, one to transfer charge, and one for
filtering. Similar to the boost converter, the charge pumps
operating frequency reduces to approximately 4kHz in
blank mode, maintaining low noise operation but at re-
duced output current capability.
Output Sequencing
Refer to the following text and Figure 1 for the LTC3450
power-up sequence. When input power is applied, the
boost converter initializes and charges its output towards
the final value of 5.1V. When the boost converter output
reaches approximately 90% of its final value (4.5V), an
internal 5V OK signal is asserted which allows the charge
pump doubler to begin operation toward its final goal of
10V. Approximately 1ms later, the charge pump inverter
begins operation toward its final goal of either
–5V or –10V depending on the connection of the V_INV
input. When the –5V or –10V output (V_NEG) reaches
approximately 50% of its final value, a 4ms (nominal)
timeout period begins. At the conclusion of the 4ms
timeout period, the charge pump tripler is allowed to
begin operation, which will eventually charge V3X to 15V
(nominal).
VX3
15V
VX2
10V
V_OUT
5V
–10V
1ms
V_NEG
4ms
3450 F01
Figure 1. Output Sequencing
3450f
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