LTC3704 Datasheet, PDF (20/28 Pages) Linear Technology – Wide Input Range, No RSENSE Positive-to-Negative DC/DC Controller

LTC3704
APPLICATIO S I FOR ATIO
regulator feedback loop acts on the resulting error amp
output signal to return V_O to its steady-state value. During
this recovery time, V_O can be monitored for overshoot or
ringing that would indicate a stability problem.
A second, more severe transient can occur when connect-
ing loads with large (> 1µF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C_O, causing a nearly instantaneous drop in V_O. No
regulator can deliver enough current to prevent this prob-
lem if the load switch resistance is low and it is driven
quickly. The only solution is to limit the rise time of the
switch drive in order to limit the inrush current di/dt to the
load.
Design Example: A 5V to 15V Input, –5V at 2A Output
Positive-to-Negative Converter
The design example presented here will be for the circuit
shown in Figure 1. The input voltage range is 5V to 15V,
and the output is -5V. The maximum load current is 2A at
an input voltage of 5V (3A peak), and 3A at an input voltage
of 15V (5A peak).
1. The maximum duty cycle of the main switch is:
D_MAX
=
V_OUT
V_OUT − V_IN(MIN)
=
–5
–10
=
50%
2. Pulse-Skip operation is chosen, so the MODE/SYNC pin
is connected to the INTV_CC pin.
3. The operating frequency is chosen to be 300kHz to
reduce the size of the inductors. From Figure 5, the resistor
from the FREQ pin to ground is 80.6k.
4. A total inductor ripple current of 40% of the maximum
is chosen, so the inductor ripple current is:
∆I_L1
=
−χ
•
I_O(MAX)
•
D_MAX
1– D_MAX
∆I_L1
=
0.4
•
2.0
•
0.5
1– 0.5
=
0.8A
For a standard 1:1 coupled inductor, the inductance is
therefore:
L1 =
L2
=
V_IN(MIN)
2 • ∆I_L1 •
f
• D_MAX
=
5
• 0.5 = 5.2µH
2 • 0.8 • 300k
The minimum saturation current for this inductor is:
I_LSAT(MIN)
≥
–^{}1+
χ₂ 
• I_O(MAX)
•
1–
1
D_MAX
= 1.2 • 2.0 • ¹ = 4.8A
1– 0.5
The inductor chosen is a BH Electronics part # 510-1009,
which has an open circuit parallel inductance of 4.56µH
and a maximum dc current rating of 6.5A.
5. For the power MOSFET,
R_DS(ON)
≤
V_SENSE(MAX)
•
₁₊
D_MAX – 1
χ
2

•
I_O(MAX)
• ρ_Τ
At the maximum duty cycle of 50%, the maximum SENSE
pin voltage is reduced to 130mV due to slope compensa-
tion, as shown in Figure 11. Assuming a maximum
junction temperature of 125°C for the power MOSFET,
ρ_Τ = 1.5, and
R_DS(ON)
≤
0.130 •
0.5 – 1
–1.2 • 2.0 •1.5
=
18.1mΩ
The MOSFET chosen was Siliconix/Vishay’s Si4884, which
has a maximum R_DS(ON) = 16.5mΩ at V_GS = 4.5V at 25°C.
The minimum BV_DSS = 30V and the maximum gate charge
is Q_G = 20nC.
6. The output diode must withstand a reverse voltage of
V_IN(MAX) – V_O = 20V and a continuous current of
I_O(MAX) = 5.0A (peak output current at V_IN = 15V). The peak
current in the diode is:
I_D(PEAK)
=
₁₊
χ
2

• I_O(MAX)
=
6A
The power dissipated in this diode at full load is:
20
sn3704 3704fs