LTC1530 Datasheet, PDF (10/24 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller

LTC1530
APPLICATIO S I FOR ATIO
Figure 5b is derived based on the condition that
I_LMAX = I_LOAD + I_RIPPLE/2. Therefore, it only provides the
minimum R_IMAX value. It must be understood that during
the initial power-up phase (V_OUT = 0V), the initial start-up
I_LMAX can be much higher than the steady state condition
I_LMAX. Therefore, R_IMAX must be selected with the start-up
I_LMAX in mind. In general, high output capacitance com-
bined with a low value inductor increases the start-up
I_LMAX. Figures 6a and 6b plot the start-up I_LMAX vs output
capacitance and inductance for unloaded and loaded con-
ditions with the current limit circuit disabled. Figures 6a
and 6b are provided as examples. Actual I_LMAX under
start-up conditions must be measured for any application
circuit so that R_IMAX can be properly chosen.
In order for the current limit circuit to operate properly and
to obtain a reasonably accurate current limit threshold, the
I_MAX and I_FB pins must be Kelvin sensed at Q1’s drain and
source pins. A 0.1µF decoupling capacitor can also be
connected across R_IMAX to filter switching noise. In addi-
tion, LTC recommends that the voltage drop across the
R_IMAX resistor be set to ≥100mV. Otherwise, noise spikes
or ringing at Q1’s source can cause the actual current limit
to be greater than the desired current limit set point.
MOSFET Gate Drive
The PV_CC supply must be greater than the input supply
voltage, V_IN, by at least one power MOSFET V_GS(ON) for
efficient operation. This higher voltage can be supplied
with a separate supply, or it can be generated using a
simple charge pump as shown in Figure 7. The 86%
maximum duty cycle ensures sufficient off-time to refresh
the charge pump during each cycle.
As PV_CC is powered up from 0V, the LTC1530 undervoltage
lockout circuit prevents G1 and G2 from pulling high until
PV_CC reaches about 3.5V. To prevent Q1’s high R_DS(ON)
from triggering the current limit comparator while PV_CC is
slewing, the current limit circuit is disabled until PV_CC is
≥ 8V. In addition, on start-up or recovery from thermal
shutdown, the driver logic is designed to hold G2 low until
G1 first goes high.
10
5500
4500
3500
R_IMAX ≥ 500Ω
I_LMAX = I_LOAD + I_RIPPLE/2
Q1 R_DS(ON) = 0.05Ω
0.04Ω
0.03Ω
2500
1500
0.02Ω
0.01Ω
500
0 2 4 6 8 10 12 14 16 18 20
I_LMAX (A)
1530 F05b
Figure 5b. Minimum Required R_IMAX vs I_LMAX
25
T_A = 25°C
V_IN = 5V
20 ILOAD = 0A
15
L = 1.2µH
10
L = 4.7µH
5
L = 2.4µH
0
02
4
6
8
10 12
OUTPUT CAPACITANCE (mF)
1530 F06a
Figure 6a. Start-Up I_LMAX vs Output Capacitance
30
T_A = 25°C
25
V_IN = 5V
I_LOAD = 10A
20
15
10
L = 2.4µH
L = 1.2µH
L = 4.7µH
5
0
024
6
8 10 12
OUTPUT CAPACITANCE (mF)
1530 F06b
Figure 6b. Start-Up I_LMAX vs Output Capacitance