LTC1267 Datasheet, PDF (9/16 Pages) Linear Technology – Dual High Efficiency Synchronous Step-Down Switching Regulators

UU W U
APPLICATIO S I FOR ATIO
1400
1200
V_OUT = 5V
1000
800
V_IN = 24V
600
V_IN = 12V
400
200
0
0
50 100 150 200 250
FREQUENCY (kHz)
LTC1267 • F03
Figure 3. Timing Capacitor Value
As the operating frequency is increased the gate charge
losses will be higher, reducing efficiency (see Efficiency
Considerations). The complete expression for operating
frequency is given by:
) ^{fO =}
1
t_OFF
1–
V_OUT
V_IN
where:
t_OFF = 1.3 × 10⁴ × C_T
Once the frequency has been set by C_T, the inductor L must
be chosen to provide no more than 0.025V/R_SENSE of
peak-to-peak inductor ripple current. This results in a
minimum required inductor value of:
L_MIN = 5.1 × 10⁵ × R_SENSE × C_T × V_OUT
As the inductor value is increased from the minimum
value, the ESR requirements for the output capacitor are
eased at the expense of efficiency. If too small an inductor
is used, the LTC1267 may not enter Burst Mode operation
and efficiency will be severely degraded at low currents.
LTC1267
LTC1267-ADJ/LTC1267-ADJ5
value, but it is very dependent on inductance selected. As
inductance increases, core losses go down but copper I²R
losses increase. For additional information regarding in-
ductor selection, please refer to the LTC1159 data sheet.
Power MOSFET and Diode Selection
Two external power MOSFETs must be selected for use
with each section of the LTC1267: a P-channel MOSFET
for the main switch, and an N-channel MOSFET for the
synchronous switch.
The peak-to-peak gate drive levels are set by the V_CC
voltage on the LTC1267. This voltage is typically 4.5V
during start-up and 5V to 7V during normal operation (see
EXT V_CC Pin Connection). Consequently, logic-level thresh-
old MOSFETs must be used in most LTC1267 family
applications. The only exceptions are applications in
which EXT V_CC is powered from an external supply greater
than 8V, in which standard threshold MOSFETs (V_GS(TH)
> 4V) may be used. Pay close attention to the BV_DSS
specification for the MOSFETs as well; many of the logic-
level MOSFETs are limited to 30V.
Selection criteria for the power MOSFETs include the on-
resistance R_DS(ON), reverse transfer capacitance C_RSS,
input voltage, and maximum output current. When the
LTC1267 is operating in continuous mode, the duty cycles
for the two MOSFETs are given by:
Duty
Cycle
=
V_OUT
V_IN
N-Channel
Duty
Cycle
=
V_IN – V_OUT
V_IN
The MOSFET dissipations at maximum output current are
given by:
Inductor Core Selection
Once the minimum value for L is known, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of more expensive
ferrite, molypermalloy (MPP), or Kool Mµ^® cores. Actual
core loss is independent of core size for a fixed inductor
Kool Mµ is a registered trademark of Magnetics, Inc.
P-Ch
P_D
=
V_OUT
V_IN
(I_MAX)²
(1
+
δ_P)
R_DS(ON)
+ k (V_IN)² (I_MAX) (C_RSS) f_O
N-Ch
P_D
=
V_IN
– V_OUT
V_IN
(I_MAX)²
(1
+
δ_N)
R_DS(ON)
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