Datasheet

LTC3809
9
3809fc
OPERATION
(Refer to Functional Diagram)
The reverse current comparator RICMP senses the drain-to-
source voltage of the bottom external N-channel MOSFET.
This MOSFET is turned off just before the inductor current
reaches zero, preventing it from going negative.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by the
voltage on the I
TH
pin. The P-channel MOSFET is turned
on every cycle (constant frequency) regardless of the I
TH
pin voltage. In this mode, the effi ciency at light loads is
lower than in Burst Mode operation. However, continuous
mode has the advantages of lower output ripple and no
noise at audio frequencies.
When the SYNC/MODE pin is clocked by an external
clock source to use the phase-locked loop (see Frequency
Selection and Phase-Locked Loop), or is set to a DC
voltage between 0.4V and several hundred mV below
V
IN
, the LTC3809 operates in PWM pulse-skipping mode
at light loads. In this mode, the current comparator
ICMP may remain tripped for several cycles and force
the external P-channel MOSFET to stay off for the same
number of cycles. 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. However, it provides
low current effi ciency higher than forced continuous
mode, but not nearly as high as Burst Mode operation.
During start-up or an undervoltage condition (V
FB
≤
0.54V), the LTC3809 operates in pulse-skipping mode
(no current reversal allowed), regardless of the state of
the SYNC/MODE pin.
Short-Circuit and Current Limit Protection
The LTC3809 monitors the voltage drop ΔV
SC
(between
the GND and SW pins) across the external N-channel
MOSFET with the short-circuit current limit comparator.
The allowed voltage is determined by:
ΔV
SC(MAX)
= A • 90mV
where A is a constant determined by the state of the IPRG
pin. Floating the IPRG pin selects A = 1; tying IPRG to V
IN
selects A = 5/3; tying IPRG to GND selects A = 2/3.
The inductor current limit for short-circuit protection is
determined by ΔV
SC(MAX)
and the on-resistance of the
external N-channel MOSFET:
I
V
R
SC
SC MAX
DS ON
=
Δ
()
()
Once the inductor current exceeds I
SC
, the short current
comparator will shut off the external P-channel MOSFET
until the inductor current drops below I
SC
.
Output Overvoltage Protection
As further protection, the overvoltage comparator (OVP)
g u a r d s a g a i n s t t r a n s i e n t o v e r s h o o t s , a s w e l l a s o t h e r m o r e
serious conditions that may overvoltage the output. When
the feedback voltage on the V
FB
pin has risen 13.33%
above the reference voltage of 0.6V, the external P-chan-
nel MOSFET is turned off and the N-channel MOSFET is
turned on until the overvoltage is cleared.