LTC3769
12
3769fa
For more information www.linear.com/LTC3769
approximately 30% of the maximum sense voltage even
though the voltage on the ITH pin indicates a lower value.
If the average inductor current is higher than the required
current, the error amplifier EA will decrease the voltage
on the ITH pin. When the ITH voltage drops below 0.425V,
the internal sleep signal goes high (enabling sleep mode)
and both external MOSFETs are turned off.
In sleep mode much of the internal circuitry is turned off
and the LTC3769 draws only 28μA of quiescent current.
In sleep mode the load current is supplied by the output
capacitor. As the output voltage decreases, the EA’s output
begins to rise. When the output voltage drops enough, the
sleep signal goes low and the controller resumes normal
operation by turning on the bottom external MOSFET on
the next cycle of the internal oscillator.
When the controller is enabled for Burst Mode operation,
the inductor current is not allowed to reverse. The reverse
current comparator (I
REV
) turns off the top external MOSFET
just before the inductor current reaches zero, preventing
it from reversing and going negative. Thus, the controller
operates in discontinuous current operation.
In forced continuous operation or when clocked by an
external clock source to use the phase-locked loop (see
the Frequency Selection and Phase-Locked Loop section),
the inductor current is allowed to reverse at light loads or
under large transient conditions. The peak inductor cur
-
rent is determined by the voltage on the ITH pin, just as
in normal operation. In this mode, the efficiency at light
loads is lower than in Burst Mode operation. However,
continuous operation has the advantages of lower output
voltage ripple and less inter
ference to audio circuitry, as
it maintains constant-frequency operation independent
of load current.
When the PLLIN/MODE pin is connected for pulse-skipping
mode, the LTC3769 operates in PWM pulse-skipping mode
at light loads. In this mode, constant-frequency operation
is maintained down to approximately 1% of designed
maximum output current. At very light loads, the current
comparator ICMP may remain tripped for several cycles
and force the external bottom MOSFET to stay off for
the same number of cycles (i.e., skipping pulses). The
inductor current is not allowed to reverse (discontinuous
operation). This mode, like forced continuous operation,
exhibits low output ripple as well as low audio noise and
reduced RF interference as compared to Burst Mode
operation. It provides higher low current efficiency than
forced continuous mode, but not nearly as high as Burst
Mode operation.
Frequency Selection and Phase-Locked Loop
(FREQ and PLLIN/MODE Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency opera
-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
The switching frequency of the LTC3769’
s controllers can
be selected using the FREQ pin.
If the PLLIN/MODE pin is not being driven by an external
clock source, the FREQ pin can be tied to SGND, tied to
INTV
CC
, or programmed through an external resistor. Tying
FREQ to SGND selects 350kHz while tying FREQ to INTV
CC
selects 535kHz. Placing a resistor between FREQ and SGND
allows the frequency to be programmed between 50kHz
and 900kHz, as shown in Figure 7.
A phase-locked loop (PLL) is available on the LTC3769
to synchronize the internal oscillator to an external clock
source that is connected to the PLLIN/MODE pin. The
LTC3769’s phase detector adjusts the voltage (through an
internal lowpass filter) of the VCO input so that the turn-on
of the external bottom MOSFET is 180° out-of-phase to
the rising edge of the external clock source. When syn
-
chronized, the LTC3769 will operate in forced continuous
mode of operation if the OVMODE pin is grounded. If the
OVMODE pin is tied to INTV
CC
, the LTC3769 will operate
in pulse-skipping mode of operation when synchronized.
The VCO input voltage is prebiased to the operating fre
-
quency set by the FREQ pin before the external clock is
applied. If prebiased near the external clock frequency,
the PLL loop only needs to make slight changes to the
VCO input in order to synchronize the rising edge of the
external clock’
s to the rising edge of BG1. The ability to
prebias the loop filter allows the PLL to lock-in rapidly
without deviating far from the desired frequency.
OPERATION
