LTC3809-1_15 Datasheet, PDF (10/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking

LTC3809-1
OPERATION ^{(Refer to Functional Diagram)}
In sleep mode, much of the internal circuitry is turned off,
reducing the quiescent current that the LTC3809-1 draws.
The load current is supplied by the output capacitor. As
the output voltage decreases, the EAMP increases the
I_TH voltage. When the I_TH voltage reaches 0.925V, the
SLEEP signal goes low and the controller resumes normal
operation by turning on the external P-channel MOSFET
on the next cycle of the internal oscillator.
When the controller is enabled for Burst Mode or pulse-
skipping operation, the inductor current is not allowed to
reverse. Hence, the controller operates discontinuously.
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 efficiency 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 MODE pin is set to the V_FB Pin, the LTC3809-1
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 audible noise
and reduced RF interference as compared to Burst Mode
operation. However, it provides low current efficiency
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 LTC3809-1
operates in pulse-skipping mode (no current reversal
allowed), regardless of the state of the MODE pin.
Short-Circuit and Current Limit Protection
The LTC3809-1 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_SC
=
ΔV_SC(MAX)
R_DS(ON)
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)
guards against transient overshoots, as well as other more
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-channel
MOSFET is turned off and the N-channel MOSFET is turned
on until the overvoltage is cleared.
38091fc
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