LTC3770_15 Datasheet, PDF (12/24 Pages) Linear Technology – Fast No RSENSE Step-Down Synchronous Controller with Margining, Tracking and PLL

LTC3770
APPLICATIONS INFORMATION
The ρ_T term is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/°C as shown in
Figure 1. For a maximum junction temperature of 100°C,
using a value ρ_T = 1.3 is reasonable.
The power dissipated by the top and bottom MOSFETs
strongly depends upon their respective duty cycles and the
load current. When the LTC3770 is operating in continuous
mode, the duty cycles for the MOSFETs are:
D_TOP
=
V_OUT
V_IN
D_BOT
=
V_IN
– V_OUT
V_IN
The resulting power dissipation in the MOSFETs at
maximum output current are:
P_TOP = D_TOP I_OUT(MAX)2 ρ_T(TOP) R_DS(ON)(MAX)
+ k V_IN2 I_OUT(MAX) C_RSS f
P_BOT = D_BOT I_OUT(MAX)2 ρ_T(BOT) R_DS(ON)(MAX)
Both MOSFETs have I²R losses and the top MOSFET includes
an additional term for transition losses, which are largest at
high input voltages. The constant k = 1.7A^–1 can be used to
estimate the amount of transition loss. The bottom MOSFET
losses are greatest when the bottom duty cycle is near 100%,
during a short-circuit or at high input voltage.
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3770 applications is
determined implicitly by the one-shot timer that controls
the on-time t_ON of the top MOSFET switch. The on-time
is set by the current out of the I_ON pin and the voltage at
the V_ON pin according to:
t_ON
=
V_VON
I_ION
(10pF)
12
Tying a resistor R_ON to SGND from the I_ON pin yields an
on-time inversely proportional to 1/3 V_IN. The current out
of the I_ON pin is:
I_ION
=
V_IN
3 R_ON
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
f
=
V_VON
•
V_OUT
3 R_ON(10pF)
[H_Z ]
To hold frequency constant during output voltage changes,
tie the V_ON pin to V_OUT. The V_ON pin has internal clamps
that limit its input to the one-shot timer. If the pin is tied
below 0.6V, the input to the one-shot is clamped at 0.6V.
Similarly, if the pin is tied above 4.8V, the input is clamped
at 4.8V. In high V_OUT applications, tie V_ON to INTV_CC. Figures
2a and 2b show how R_ON relates to switching frequency
for several common output voltages.
1000
V_OUT = 3.3V
V_OUT = 2.5V
V_OUT = 1.5V
100
100
R_ON (kΩ)
1000
3770 F02a
Figure 2a. Switching Frequency vs R_ON (V_ON = 0V)
1000
V_OUT = 12V
V_OUT = 3.3V
V_OUT = 5V
100
10
100
R_ON (kΩ)
1000
3770 F02b
Figure 2b. Switching Frequency vs R_ON (V_ON = INTV_CC)
3770fc