LTC3523-2_15 Datasheet, PDF (9/16 Pages) Linear Technology – Synchronous 600mA Step-Up and 400mA Step-Down DC/DC Converters

LTC3523/LTC3523-2
OPERATION
Step-Up Anti-Ringing Control
The anti-ring circuitry connects a resistor across the in-
ductor to prevent high frequency ringing on the SW1 pin
during discontinuous current mode operation. The ringing
of the resonant circuit formed by L and C_SW (capacitance
on SW pin) is low energy, but can cause EMI radiation.
Step-Up Output Disconnect
The LTC3523/LTC3523-2 step-up is designed to provide
true output disconnect by eliminating body diode conduc-
tion of the internal P-channel MOSFET rectifier. This allows
for V_OUT to go to zero volts during shutdown, drawing no
current from the input source. Controlling the P-channel
MOSFET body diode also enables inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output dis-
connect, an external Schottky diode cannot be connected
between SW1 and V_OUT.
Thermal Shutdown
If the die temperature reaches 160°C, the part will go into
thermal shutdown. All switches will be turned off and
the soft-start capacitor will be discharged. The device
will be enabled again when the die temperature drops by
approximately 15°C.
APPLICATIONS INFORMATION
PCB LAYOUT GUIDELINES
The high speed operation of the LTC3523/LTC3523-2
demands careful attention to board layout. You will not
get advertised performance with careless layout. Figure 1
shows the recommended component placement. A large
ground pin copper area will help to lower the chip tem-
perature. A multilayer board with a separate ground plane
is ideal, but not absolutely necessary.
COMPONENT SELECTION
Inductor Selection
The LTC3523/LTC3523-2 can utilize small surface mount
and chip inductors due to its fast 1.2MHz switching
frequency and for the 2.4MHz version, the values are
halved. The Inductor current ripple is typically set for
20% to 40% of the peak inductor current (I_P). High
Figure 1. Recommended Component Placement for Double Layer Board
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