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LTC3558

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 efﬁcien-

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 (PVIN1) should be connected to the

battery pin (BAT) and PVIN2. 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 PVIN1 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:

VOUT = 0.8(1 + R1/R2)

Typical values for R1 are in the range of 40k to 1M. The

capacitor CFB 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 CFB 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

MODE

0.8V

CFB

VOUT

PVIN

3558 F08

Figure 8. Buck Converter Application Circuit

LTC3558 Datasheet(PDF) 21 Page - Linear Technology

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