LTC1159-3.3_15 Datasheet, PDF (7/20 Pages) Linear Technology – High Efficiency Synchronous Step-Down Switching Regulators

U
OPERATIO (Refer to Functional Diagram)
increases, the output voltage decreases slightly. This causes
the output of the gain stage to increase the current com-
parator threshold, thus tracking the load current.
The sequence of events for Burst Mode operation is very
similar to continuous operation with the cycle interrupted
by the voltage comparator. When the output voltage is at or
above the desired regulated value, the P-channel MOSFET
is held off by comparator V and the timing capacitor con-
tinues to discharge below V_TH1. When the timing capacitor
discharges past V_TH2, voltage comparator S trips, causing
the internal SLEEP line to go low and the N-channel MOSFET
to turn off.
The circuit now enters sleep mode with both power
MOSFETs turned off. In sleep mode, much of the circuitry
LTC1159
LTC1159-3.3/LTC1159-5
is turned off, dropping the supply current from several
milliamps (with the MOSFETs switching) to 300µA. When
the output capacitor has discharged by the amount of
hysteresis in comparator V, the P-channel MOSFET is
again turned on and this process repeats. To avoid the
operation of the current loop interfering with Burst Mode
operation, a built-in offset is incorporated in the gain stage.
To prevent both the external MOSFETs from being turned
on at the same time, feedback is incorporated to sense the
state of the driver output pins. Before the N-gate output can
go high, the P-drive output must also be high. Likewise, the
P-drive output is prevented from going low when the
N-gate output is high.
APPLICATIO S I FOR ATIO
The LTC1159 Compared to the LTC1148/LTC1149
Families
The LTC1159 family is closest in operation to the LTC1149
and shares much of the applications information. In addi-
tion to reduced quiescent and shutdown currents, the
LTC1159 adds an internal switch which allows the driver
and control sections to be powered from an external
source for higher efficiency. This change affects Power
MOSFET Selection, EXTV_CC Pin Connection, Important
Information About LTC1159 Adjustable Applications, and
Efficiency Considerations found in this section.
The basic LTC1159 application circuit shown in Figure 1
is limited to a maximum input voltage of 30V due to
MOSFET breakdown. If the application does not require
greater than 18V operation, then the LTC1148 or
LTC1148HV should be used. For higher input voltages
where quiescent and shutdown current are not critical, the
LTC1149 may be a better choice since it is set up to drive
standard threshold MOSFETs.
R_SENSE Selection for Output Current
R_SENSE is chosen based on the required output current. The
LTC1159 current comparator has a threshold range that
extends from a minimum of 0.025V/R_SENSE to a maximum
of 0.15V/R_SENSE. The current comparator threshold sets
the peak of the inductor ripple current, yielding a maximum
output current I_MAX equal to the peak value less half the
peak-to-peak ripple current. For proper Burst Mode opera-
tion, I_RIPPLE(P-P) must be less than or equal to the minimum
current comparator threshold.
Since efficiency generally increases with ripple current,
the maximum allowable ripple current is assumed, i.e.,
I_RIPPLE(P-P) = 0.025V/R_SENSE (see C_T and L Selection for
Operating Frequency). Solving for R_SENSE and allowing
a margin for variations in the LTC1159 and external
component values yields:
R_SENSE
=
100
I_MAX
mΩ
A graph for selecting R_SENSE versus maximum output
current is given in Figure 2. The LTC1159 series works well
with values of R_SENSE from 0.02Ω to 0.2Ω.
The load current below which Burst Mode operation com-
mences, I_BURST, and the peak short-circuit current, I_SC(PK),
both track I_MAX. Once R_SENSE has been chosen, I_BURST and
I_SC(PK) can be predicted from the following equations:
7