Gate Drive and High-Side N-FETs

Part 6 · Voltage Regulation

Gate drive

Key point: a high-side N-FET source sits on SW. \(C_{\mathrm{BST}}\) charges from INTVCC while SW is low, then rides up with SW, so the driver still has a VCC-sized rail when the source is at VIN.

\(V(C_{\mathrm{BST}}) = V_{\mathrm{CC}} - V_f\). BOOST sits that far above SW. With HO on, the gate is at BOOST, so \(V_{GS}\) is the brick.

1
SW low. Charge the brick.
12 V buck · 5 V VCC, \(V_f = 0.60~\mathrm{V}\). Scene is charge. SW is \(0.00~\mathrm{V}\). BOOST is \(4.40~\mathrm{V}\). \(V(C_{\mathrm{BST}})\) is \(4.40~\mathrm{V}\). HS \(V_{GS}\) is \(0.00~\mathrm{V}\). Diode is forward · charging. The figure shows LO on.
2
SW at VIN. The brick rides up.
Same rails. Scene is ride. SW is \(12.0~\mathrm{V}\). BOOST is \(16.4~\mathrm{V}\). \(V(C_{\mathrm{BST}})\) is \(4.40~\mathrm{V}\). HS \(V_{GS}\) is \(4.40~\mathrm{V}\). Diode is reverse · blocking.
3
24 V. Same brick, higher BOOST.
24 V preset, still ride. SW is \(24.0~\mathrm{V}\). BOOST is \(28.4~\mathrm{V}\). \(V(C_{\mathrm{BST}})\) is \(4.40~\mathrm{V}\). HS \(V_{GS}\) is \(4.40~\mathrm{V}\). Diode is reverse · blocking.
4
48 V preset. Charge from 6 V.
48 V · 6 V VCC, \(V_f = 0.60~\mathrm{V}\). Scene is charge. SW is \(0.00~\mathrm{V}\). BOOST is \(5.40~\mathrm{V}\). \(V(C_{\mathrm{BST}})\) is \(5.40~\mathrm{V}\). HS \(V_{GS}\) is \(0.00~\mathrm{V}\). Diode is forward · charging.
5
48 V ride. \(V_{GS}\) is the brick.
Same rails. Scene is ride. SW is \(48.0~\mathrm{V}\). BOOST is \(53.4~\mathrm{V}\), and the HS gate is on that rail. \(V(C_{\mathrm{BST}})\) is \(5.40~\mathrm{V}\). HS \(V_{GS}\) is \(5.40~\mathrm{V}\). Diode is reverse · blocking.

Ideal switches, instant edges, no \(C_{\mathrm{BST}}\) droop from \(Q_g\). The diode conducts while SW is low, so a 100% duty cycle never refreshes the cap. HO is the amp between BOOST and SW.

Bootstrap · charge, then ride up

\(V_{\mathrm{IN}}\) (V) INTVCC (V) Diode \(V_f\) (V)

Half-bridge · HO between BOOST and SW

One cycle · volts vs ground

SW—

BOOST—

V(C_BST)—

HS \(V_{GS}\)—

Diode—

\(V(C_{\mathrm{BST}}) = V_{\mathrm{CC}} - V_f\)

→ —

\(V_{\mathrm{BOOST}} = V_{\mathrm{SW}} + V(C_{\mathrm{BST}})\)

→ —

\(V_{GS} = V_{\mathrm{GATE}} - V_{\mathrm{SW}}\)

→ —