LTC3602 datasheet(15/28 Pages) LINER

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LTC3633A-2/LTC3633A-3

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Figure 3. Setting the Output Voltage

Figure 2. Diode-OR Circuit

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VOUT

SGND

LTC3633A-2

APPLICATIONS INFORMATION

Furthermore, as long as SVIN is powered, the LTC3633A-2/

LTC3633A-3 operates as a step-down regulator with PVIN

voltages as low as 1.5V (subject to minimum off-time

constraints). However, at PVIN voltages less than 3V, in-

ternal on-time calculation errors increase, and controlled

on-time operation is not guaranteed. If this occurs, the

output voltages will remain in regulation, but the switch-

ing frequency of each channel may deviate from the

programmed frequency under these conditions and phase

lock between the two channels may be lost.

Boost Capacitor

The LTC3633A-2 uses a “bootstrap” circuit to create a

voltage rail above the applied input voltage PVIN. Speciﬁ-

cally, a boost capacitor, CBOOST, is charged to a voltage

approximately equal to INTVCC each time the bottom power

MOSFET is turned on. The charge on this capacitor is then

used to supply the required transient current during the

remainder of the switching cycle. When the top MOSFET

is turned on, the BOOST pin voltage will be equal to ap-

proximately PVIN + 3.3V. For most applications, a 0.1μF

ceramic capacitor closely connected between the BOOST

and SW pins will provide adequate performance.

Output Voltage Programming

Each regulator’s output voltage is set by an external resis-

tive divider according to the following equation:

VOUT = 0.6V 1+

⎛

⎝

⎜

⎞

⎠

⎟

The desired output voltage is set by appropriate selection

of resistors R1 and R2 as shown in Figure 3. Choosing

large values for R1 and R2 will result in improved zero-

load efﬁciency but may lead to undesirable noise coupling

or phase margin reduction due to stray capacitances

at the VFB node. Care should be taken to route the VFB

trace away from any noise source, such as the SW trace.

To improve the frequency response of the main control

loop, a feedforward capacitor, CF , may be used as shown

in Figure 3.

Connecting the VON pin to the output voltage makes the

on-time proportional the output voltage and allows the

internal on-time servo loop to lock the converter’s switching

frequency to the programmed value. If the output voltage

is outside the VON sense range (0.6V – 6V for LTC3633A-2,

1.5V – 12V for LTC3633A-3), the output voltage will stay

in regulation, but the switching frequency may deviate

from the programmed frequency.

Minimum Off-Time/On-Time Considerations

The minimum off-time is the smallest amount of time that

the LTC3633A-2 can turn on the bottom power MOSFET,

trip the current comparator and turn the power MOSFET

back off. This time is typically 45ns. For the controlled

on-time architecture, the minimum off-time limit imposes

a maximum duty cycle of:

DC(MAX) = 1– f • tOFF(MIN) + 2• tDEAD

()

where f is the switching frequency, tDEAD is the nonoverlap

time, or “dead time” (typically 10ns) and tOFF(MIN) is the

minimum off-time. If the maximum duty cycle is surpassed,

due to a dropping input voltage for example, the output

will drop out of regulation. The minimum input voltage to

avoid this dropout condition is:

VIN(MIN) =

VOUT

1− f• tOFF(MIN) + 2• tDEAD

()

SUPPLY1

PVIN1

PVIN2

SVIN

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SUPPLY2

LTC3633A-2

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