Datasheet
ADP1853 Data Sheet
Rev. 0 | Page 22 of 28
Setting the Current Sense Gain
The voltage drop across the external low-side MOSFET is
sensed by a current sense amplifier by multiplying the peak
inductor current and the R
DSON
of the MOSFET. The result is
then amplified by a gain factor of 3 V/V, 6 V/V, or 12 V/V,
which is programmable by an external resistor, R
CSG
, connected
to the DL pin. This gain is sensed only during power-up and
not during normal operation. The amplified voltage is summed
with the slope compensation ramp voltage and fed into the
PWM controller for a stable regulation voltage.
The voltage range of the internal node, V
CS
, is between 0.4 V
and 2.2 V. Select the current sense gain such that the internal
minimum amplified voltage (V
CSMIN
) is above 0.4 V and the
maximum amplified voltage (V
CSMAX
) is 2.1 V. Note that V
CSMIN
or V
CSMAX
is not the same as V
COMP
, which has a range of 0.85 V
to 2.2 V. Make sure that the maximum V
COMP
(V
COMPMAX
) does
not exceed 2.2 V to account for temperature and part-to-part
variations. See the following equations for V
CSMIN
, V
CSMAX
, and
V
COMPMAX
:
CSMINDSON
L
CSMIN
ARIV
_
2
1
V75.0
CSMAXDSON
L
LOADMAXCSMAX
ARIIV
_
)
2
1
(V75.0
CSMAX
RAMP
ON
IN
COMPMAX
V
R
tV
V
pF100
)V2.0(
where:
V
CSMIN
is the minimum amplified voltage of the internal current
sense amplifier at zero output current.
I
L
is the peak-to-peak ripple current in the inductor.
R
DSON_MIN
is the low-side MOSFET minimum on resistance. The
zero current level voltage of the current sense amplifier is 0.75 V.
V
CSMAX
is the maximum amplified voltage of the internal current
sense amplifier at the maximum output current.
I
LOADMAX
is the maximum output dc load current.
V
COMPMAX
is the maximum voltage at the COMP pin.
t
ON
is the high-side driver (DH) on time.
Replace R
DSON
with the resistance value of the current sense
element, R
CS
, if it is used.
Type II Compensation
Figure 29. Type II Compensation
In this case, use the circuit shown in Figure 29. Calculate the
compensation resistor, R
Z
, with the following equation:
CO
OUT
S
TOPZ
fCRRR
2
(11)
where:
f
CO
is chosen to be 1/10 of f
SW
.
R
S
= A
CS
× R
DSON_MIN
.
A
CS
is the current sense gain of either 3 V/V, 6 V/V, or 12 V/V,
set by the gain resistor between DL and PGND.
R
DSON_MIN
is the low-side MOSFET minimum on resistance.
If the current is sensed on a current sense resistor, R
CS
, then R
CS
becomes
CSCSS
RAR
Next, choose the compensation capacitor to set the compensa-
tion zero, f
Z1
, to the lesser of 1/5 of the crossover frequency or ½
of the LC resonant frequency
I
Z
SWCO
Z1
CR
ff
f
2
1
505
(12)
or
I
Z
LC
Z1
CR
f
f
2
1
2
(13)
Solving for C
I
in Equation 12 yields
SW
Z
I
fR
C
25
(14)
Solving for C
I
in Equation 13 yields
LC
Z
I
fR
C
1
(15)
G
(dB)
PHASE
–180°
–270°
f
Z
f
P
C
HF
C
I
R
Z
R
TOP
R
BOT
V
OUT
INTERNAL
VREF
EA
FB
COMP
–
1
S
L
O
P
E
–
1
S
L
O
P
E
10594-035