ADP1853 Datasheet, PDF (20/28 Pages) Analog Devices – Synchronous, Step-Down DC-to-DC Controller

ADP1853
There is also additional power loss during the time, known as
dead time, between the turn-off of the high-side switch and the
turn-on of the low-side switch, when the body diode of the low-
side MOSFET conducts the output current. The power loss in
the body diode is given by
P_BODYDIODE = V_F × t _D × f _SW × I_O
where:
V_F is the forward voltage drop of the body diode, typically 0.7 V.
t_D is the dead time in the ADP1853, typically 30 ns when
driving a medium size MOSFETs with input capacitance, C_iss,
of approximately 3 nF. The dead time is not fixed. Its effective
value varies with gate drive resistance and C_iss; therefore,
P_BODYDIODE increases in high load current designs and low voltage
designs.
Then the power loss in the low-side MOSFET is
P_LS = P_CLS + P_BODYDIODE
Note that MOSFET on resistance, R_DSON, increases with
increasing temperature with a typical temperature coefficient of
0.4%/^oC. The MOSFET junction temperature (T_J) rise over the
ambient temperature is
T_J = T_A + θ_JA × P_D
where:
θ_JA is the thermal resistance of the MOSFET package.
T_A is the ambient temperature.
P_D is the total power dissipated in the MOSFET.
LOOP COMPENSATION—VOLTAGE MODE
Set the controller to voltage mode operation by placing a
100 kΩ resistor between DL and PGND. Chose the larger
possible ramp amplitude for the voltage mode below 1.5 V.
The ramp voltage is programmed by a resistor value between
V_IN and the RAMP pin:
R_RAMP
=
100
V_IN
pF ×
− 0.2 V
f_SW ×V_RAMP
The voltage at the RAMP pin is fixed at 0.2 V, and the current
going into RAMP should be between 10 µA and 160 µA. Make
sure that the following condition is satisfied:
10 μA ≤ ^{VIN − 0.2 V} ≤ 160 μA
(1)
R_RAMP
For instance, with an input voltage of 12 V, R_RAMP should not be
less than 73.8 kΩ.
Assuming that the LC filter design is complete, the feedback
control system can be compensated. In general, aluminum
electrolytic capacitors have high ESR; however, if several
aluminum electrolytic capacitors are connected in parallel and
produce a low effective ESR, then Type III compensation is
needed. In addition, ceramic capacitors have very low ESR (only
a few milliohms) making Type III compensation a better choice.
Data Sheet
Type III Compensation
G
(dB)
–1 _SLOPE
+1 ^{SLOPE –1 SLOPE}
–90°
PHASE
f_Z
f_P
–270°
C_HF
R_FF C_FF
R_Z
C_I
V_OUT
R_TOP
R_BOT
FB EA
COMP
INTERNAL
VREF
Figure 27. Type III Compensation
If the output capacitor ESR zero frequency is greater than ½ of
the crossover frequency, use the Type III compensator as shown
in Figure 27.
Calculate the output LC filter resonant frequency as follows:
f _LC
=
2π
1
LC
(2)
Chose a crossover frequency that is 1/10 of the switching
frequency:
f_CO
=
f_SW
10
(3)
Set the poles and zeros as follows:
f _P1
=
f _P2
=
1
2
f_SW
(4)
f_Z1 =
f _Z2
=
f_CO
4
=
f_SW
40
=¹
2πR_ZC_I
(5)
or
f_Z1 =
f _Z2
=
f _LC
2
=
1
2πR_ZC_I
(6)
Use the lower zero frequency from Equation 5 or Equation 6.
Calculate the compensator resistor, R_Z, as follows:
R_Z
=
R_TOPV_RAMP f _Z1 f _CO
V_IN f _LC ²
(7)
Next, calculate C_I:
C_I
=
1
2πR_Z
f _Z1
(8)
Because of the finite output current drive of the error amplifier,
C_I needs to be less than 10 nF. If it is larger than 10 nF, choose a
larger R_TOP and recalculate R_Z and C_I until C_I is less than 10 nF.
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