LTC3558
21
3558f
APPLICATIONS INFORMATION
Buck Switching Regulator General Information
The LTC3558 contains a 2.25MHz constant-frequency
current mode buck switching regulator that can provide
up to 400mA. The switcher can be programmed for a
minimum output voltage of 0.8V and can be used to power
a microcontroller core, microcontroller I/O, memory or
other logic circuitry. The regulator supports 100% duty
cycle operation (dropout mode) when the input voltage
drops very close to the output voltage and is also capable
of operating in Burst Mode operation for highest effi cien-
cies at light loads (Burst Mode operation is pin selectable).
The buck switching regulator also includes soft-start to
limit inrush current when powering on, short-circuit cur-
rent protection, and switch node slew limiting circuitry to
reduce radiated EMI.
A MODE pin sets the buck switching regulator in Burst
Mode operation or pulse skip operating mode. The regula-
tor is enabled individually through its enable pin. The buck
regulator input supply (PV
IN1
) should be connected to the
battery pin (BAT) and PV
IN2
. This allows the undervoltage
lockout circuit on the BAT pin to disable the buck regulators
when the BAT voltage drops below 2.45V. Do not drive the
buck switching regulator from a voltage other than BAT.
A 10μF decoupling capacitor from the PV
IN1
pin to GND
is recommended.
Buck Switching Regulator
Output Voltage Programming
The buck switching regulator can be programmed for
output voltages greater than 0.8V. The output voltage
for the buck switching regulator is programmed using a
resistor divider from the switching regulator output con-
nected to its feedback pin (FB1), as shown in Figure 8,
such that:
V
OUT
= 0.8(1 + R1/R2)
Typical values for R1 are in the range of 40k to 1M. The
capacitor C
FB
cancels the pole created by feedback re-
sistors and the input capacitance of the FB pin and also
helps to improve transient response for output voltages
much greater than 0.8V. A variety of capacitor sizes can
be used for C
FB
but a value of 10pF is recommended for
most applications. Experimentation with capacitor sizes
between 2pF and 22pF may yield improved transient
response if so desired by the user.
Buck Switching Regulator Operating Modes
The buck switching regulator includes two possible oper-
ating modes to meet the noise/power needs of a variety
of applications.
In pulse skip mode, an internal latch is set at the start of
every cycle, which turns on the main P-channel MOSFET
PWM
CONTROL
GND
EN
MODE
0.8V
MN
FB
MP
C
FB
V
OUT
P
VIN
C
O
R1
R2
L
SW
3558 F08
Figure 8. Buck Converter Application Circuit
