Datasheet
ADP1853 Data Sheet
Rev. 0 | Page 20 of 28
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
OSW
DF
BODYDIODE
IftVP ×××=
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
BODYDIODECLSLS
PPP +=
Note that MOSFET on resistance, R
DSON
, increases with
increasing temperature with a typical temperature coefficient of
0.4%/
o
C. 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:
RAMPSW
IN
RAMP
Vf
V
R
××
−
=
pF100
V2.0
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:
μA160
V2.0
μA10 ≤
−
≤
RAMP
IN
R
V
(1)
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.
Type III Compensation
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:
LCπ
f
LC
2
1
=
(2)
Chose a crossover frequency that is 1/10 of the switching
frequency:
10
SW
CO
f
f =
(3)
Set the poles and zeros as follows:
SW
P2P1
fff
2
1
==
(4)
I
Z
SWCO
Z2Z1
CR
ff
ff
π
2
1
404
====
(5)
or
I
Z
LC
Z2Z1
CR
f
ff
π
2
1
2
===
(6)
Use the lower zero frequency from Equation 5 or Equation 6.
Calculate the compensator resistor, R
Z
, as follows:
2
LC
IN
CO
Z1
RAMP
TOP
Z
fV
ffVR
R =
(7)
Next, calculate C
I
:
Z1Z
I
fR
C
π
=
2
1
(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.
G
(dB)
PHASE
–90°
–270°
f
Z
f
P
C
HF
C
I
R
Z
R
FF
R
TOP
R
BOT
V
OUT
INTERNAL
VREF
EA
FB
COMP
–1 SLOPE
–1 SLOPE
C
FF
+1 SLOPE
10594-033