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LTC3554

3554f

STEP-DOWN SWITCHING REGULATOR

Introduction

The LTC3554 includes two constant-frequency cur-

rent-mode 200mA step-down switching regulators, also

known as buck regulators. At light loads, each regulator

automatically enters Burst Mode operation to maintain

high efﬁciency.

Applications with a near-zero-current sleep or memory

keep-alive mode can command the LTC3554 switching

regulators into a standby mode that maintains output

regulation while drawing only 1.5μA quiescent current

per active regulator. Load capability drops to 5mA per

regulator in this mode.

Switching frequency and switch slew rate are pin-select-

able, allowing the application circuit to dynamically trade

off efﬁciency and EMI performance.

The regulators are enabled, disabled and sequenced

through the pushbutton interface (see the Pushbutton

Interface section for more information). It is recommended

that the step-down switching regulator input supply

(BVIN) be connected to the system supply pin (VOUT).

This is recommended because the undervoltage lockout

circuit on the VOUT pin (VOUT UVLO) disables the step-

down switching regulators when the VOUT voltage drops

below the VOUT UVLO threshold. If driving the step-down

switching regulator input supplies from a voltage other

than VOUT, the regulators should not be operated outside

their speciﬁed operating voltage range as operation is not

guaranteed beyond this range.

Output Voltage Programming

Figure 3 shows the step-down switching regulator

application circuit. The output voltage for each step-down

switching regulator is programmed using a resistor divider

from the step-down switching regulator output connected

to the feedback pins (FB1 and FB2) such that:

VOUTx = 0.8V •

Typical values for R1 can be as high as 2.2MΩ.

(R1 + R2) can be as high as 3MΩ. The capacitor CFB

cancels the pole created by feedback resistors 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.

OPERATION

Figure 3. Step-down Switching Regulator

Application Circuit

3554 F03

0.8V

VIN

CFB

COUT

VOUTx

GND

FBx

SWx

PWM

CONTROL

FSEL

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