ADP1853 Datasheet, PDF (22/28 Pages) Analog Devices – Synchronous, Step-Down DC-to-DC Controller

ADP1853
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 RDSON 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, RCSG, 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, VCS, is between 0.4 V
and 2.2 V. Select the current sense gain such that the internal
minimum amplified voltage (VCSMIN) is above 0.4 V and the
maximum amplified voltage (VCSMAX) is 2.1 V. Note that VCSMIN
or VCSMAX is not the same as VCOMP, which has a range of 0.85 V
to 2.2 V. Make sure that the maximum VCOMP (VCOMPMAX) does
not exceed 2.2 V to account for temperature and part-to-part
variations. See the following equations for VCSMIN, VCSMAX, and
VCOMPMAX:
V_CSMIN

0 .75
V

1
2
I_L
 R_DSON
_ MIN
 A_CS
V_CSMAX
 0.75 V (I_LOADMAX

1
2
I_L
)

R_DSON
_ MAX
 A_CS
V_COMPMAX

(V_IN  0.2 V)t_ON
100 pF  R_RAMP
 V_CSMAX
where:
VCSMIN is the minimum amplified voltage of the internal current
sense amplifier at zero output current.
IL is the peak-to-peak ripple current in the inductor.
RDSON_MIN is the low-side MOSFET minimum on resistance. The
zero current level voltage of the current sense amplifier is 0.75 V.
VCSMAX is the maximum amplified voltage of the internal current
sense amplifier at the maximum output current.
ILOADMAX is the maximum output dc load current.
VCOMPMAX is the maximum voltage at the COMP pin.
tON is the high-side driver (DH) on time.
Replace RDSON with the resistance value of the current sense
element, RCS, if it is used.
Data Sheet
Type II Compensation
G
(dB)
–1 _SLOPE
PHASE
f_Z
–180°
–270°
–1 _SLOPE
f_P
C_HF
R_Z
C_I
V_OUT ^RTOP
R_BOT ^FB
EA
COMP
INTERNAL
VREF
Figure 29. Type II Compensation
In this case, use the circuit shown in Figure 29. Calculate the
compensation resistor, RZ, with the following equation:
R_Z  R_TOP  R_S 2 C_OUT f_CO
(11)
where:
fCO is chosen to be 1/10 of fSW.
RS = ACS × R . DSON_MIN
ACS 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.
RDSON_MIN is the low-side MOSFET minimum on resistance.
If the current is sensed on a current sense resistor, RCS, then RCS
becomes
R_S  A_CS  R_CS
Next, choose the compensation capacitor to set the compensa-
tion zero, fZ1, to the lesser of 1/5 of the crossover frequency or ½
of the LC resonant frequency
f_Z1 
f_CO
5

f_SW
50
¹
2R_ZC_I
(12)
or
f_Z1 
f _LC
2
¹
2R_ZC_I
(13)
Solving for CI in Equation 12 yields
C_I

25
R_Z f_SW
(14)
Solving for CI in Equation 13 yields
C_I

1
R_Z f_LC
(15)
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