MAX15108 Datasheet, PDF (14/19 Pages) Maxim Integrated Products – High-Efficiency, 8A, Current-Mode Synchronous Step-Down Switching Regulator

High-Efficiency, 8A, Current-Mode
Synchronous Step-Down Switching Regulator
During tON and tOFF1, the output capacitor stores a
charge equal to:
∆Q_OUT
=
L × (I_SKIP-LIMIT
-
I_LOAD
)
2
×



V_IN
2
1
- V_OUT
+
1
V_OUT



During tOFF2 (= n x tCK, number of clock cycles
skipped), the output capacitor loses this charge:
t _OFF2
=
∆Q _OUT
I_LOAD
→
t _OFF2
=
L × (I_SKIP-LIMIT
-
I_LOAD
)
2
×



V_IN
2 × I_LOAD
1
- V_OUT
+
1
V_OUT



Finally, frequency in skip mode is:
f_SKIP
=
t _ON
+
1
t _OFF1
+
t _OFF2
Output ripple in skip mode is:
V_OUT-RIPPLE = V_COUT-RIPPLE + V_ESR-RIPPLE =
(I_SKIP-LIMIT - I_LOAD) ×
C_OUT
t _ON
+
R_ESR,COUT
×
(I_SKIP-LIMIT
- I_LOAD)
V_OUT-RIPPLE =
 _L×
_COUT
I_SKIP-LIMIT
× (V_IN - V_OUT
)
+

R _{ESR,COUT }

×
(I_SKIP-LIMIT
- I_LOAD)
Size COUT based on the above formula to limit output
ripple in skip mode.
Compensation Design Guidelines
The IC uses a fixed-frequency, peak-current-mode con-
trol scheme to provide easy compensation and fast tran-
sient response. The inductor peak current is monitored
on a cycle-by-cycle basis and compared to the COMP
voltage (output of the voltage error amplifier). The regu-
lator’s duty cycle is modulated based on the inductor’s
peak current value. This cycle-by-cycle control of the
inductor current emulates a controlled current source.
As a result, the inductor’s pole frequency is shifted
beyond the gain bandwidth of the regulator. System
stability is provided with the addition of a simple series
capacitor-resistor from COMP to PGND. This pole-zero
combination serves to tailor the desired response of the
closed-loop system. The basic regulator loop consists
of a power modulator (comprising the regulator’s pulse-
width modulator, compensation ramp, control circuitry,
MOSFETs, and inductor), the capacitive output filter
and load, an output feedback divider, and a voltage-
loop error amplifier with its associated compensation
circuitry. See Figure 1.
The average current through the inductor is expressed
as:
I_L = G_MOD × V_COMP
where IL is the average inductor current and GMOD is
the power modulator’s transconductance.
For a buck converter:
V_OUT = R_LOAD × I_L
where RLOAD is the equivalent load resistor value.
Combining the above two relationships, the power mod-
ulator’s transfer function in terms of VOUT with respect
to VCOMP is:
V_OUT
V_COMP
=
R_LOAD
I_L
× I_L
= R_LOAD
× G_MOD
G_MOD
Having defined the power modulator’s transfer function
gain, the total system loop gain can be written as follows
(see Figure 1):
α
=
R_OUT × (sC_CR_C + 1)
s₍C_C + C_CC)(R_C + R_OUT)
+ 1
×
s₍C_C || C_CC)(R_C || R_OUT) + 1
β
=
G_MOD
×
R_LOAD
×
(sC_OUTESR + 1)
sC_{OUT (}ESR + R_LOAD)
+
1
Gain = ^R2 × ^{A VEA} × α × β
R₁ + R₂ R_OUT
where ROUT is the quotient of the error amplifier’s DC
gain, AVEA, divided by the error amplifier’s transconduc-
tance, gMV; ROUT is much larger than RC.
R_{2 =} V_FB
R₁ + R₂ V_OUT
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