LTC1514 查看數據表（PDF） - Linear Technology

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LTC1514

Step-Up/Step-Down Switched Capacitor DC/DC Converters with Low-Battery Comparator

Linear Technology 

LTC1514-3.3/LTC1514-5

PIN FUNCTIONS

battery comparator and 1.145V reference are kept alive in

shutdown.

GND (Pin 4): Ground. Should be tied to a ground plane for

best performance.

C1– (Pin 5): Charge Pump Flying Capacitor, Negative

Terminal.

C1+ (Pin 6): Charge Pump Flying Capacitor, Positive

Terminal.

VIN (Pin 7): Charge Pump Input Voltage. May be between

2V and 8V (LTC1514-3.3) or between 2.7V and 10V

(LTC1514-5). VIN should be bypassed with a ≥ 10µF low

ESR capacitor as close as possible to the pin for best

performance.

VOUT (Pin 8): Regulated Output Voltage. The output volt-

age is internally set to either 3.3V (LTC1514-3.3) or to 5V

(LTC1514-5) using an internal resistor divider. VOUT should

be bypassed with a ≥ 10µF low ESR capacitor as close as

possible to the pin for best performance.

APPLICATIONS INFORMATION

Regulator Operation

The regulator section of the LTC1514-3.3/LTC1514-5

consists of a charge pump, reference, comparator and

some logic. The divided down output voltage is com-

pared to the internal reference voltage. When the divided

output drops below the reference voltage, the charge

pump is enabled, which boosts the output back into

regulation. Hysteresis in the comparator forces the regu-

lator to burst on and off and causes approximately

100mV of peak-to-peak ripple to appear at the output. By

enabling the charge pump only when needed, the

LTC1514-3.3 and LTC1514-5 are able to achieve high

efficiencies with low output load currents.

Each part’s charge pump has a unique architecture that

allows the input voltage to be either stepped up or

stepped down to produce a regulated output. Internal

circuitry senses the VIN to VOUT differential voltage and

controls the charge pump operating mode. In addition,

the effective output impedance of the charge pump is

internally adjusted to prevent large inrush currents and

allow for a wide input voltage range. When the input

voltage is lower than the output voltage, the charge pump

operates as a step-up voltage doubler. When the input

voltage is greater than the output, the charge pump

operates as a step-down gated switch.

Capacitor Selection

For best performance, low ESR capacitors are recom-

mended for both CIN and COUT to reduce noise and ripple.

The CIN and COUT capacitors should be either ceramic or

tantalum and should be 10µF or greater. If the input

source impedance is very low (< 0.5Ω), CIN may not be

needed. Increasing the size of COUT to 22µF or greater will

reduce output voltage ripple—particularly with high VIN

voltages (8V or greater). A ceramic capacitor is recom-

mended for the flying capacitor C1 with a value of 0.1µF

or 0.22µF. Smaller value flying capacitors may be used in

low output current applications.

Output Ripple

Normal LTC1514-3.3/LTC1514-5 operation produces

voltage ripple on the VOUT pin. Output voltage ripple is

required for the parts to regulate. Low frequency ripple

exists due to the hysteresis in the sense comparator and

propagation delays in the charge pump enable/disable

circuits. High frequency ripple is also present mainly

from the ESR (equivalent series resistance) in the output

capacitor. Typical output ripple (VIN < 8V) under maxi-

mum load is 100mV peak-to-peak with a low ESR (< 0.5Ω)

10µF output capacitor. For applications requiring VIN to

exceed 8V, a 22µF or larger COUT capacitor is recom-

mended to maintain max ripple in the 100mV range.

The magnitude of the ripple voltage depends on several

factors. High input voltages increase the output ripple

since more charge is delivered to COUT per charging

cycle. A large C1 flying capacitor (> 0.22µF) also

increases ripple in step-up mode for the same reason.

Large output current load and/or a small output capacitor

(< 10µF) results in higher ripple due to higher output

voltage dV/dt. High ESR capacitors (ESR > 0.5Ω) on the

5

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